Gem Info (Gem Testing)

Commonly Encountered Gemstones

Download this document in PDF

A mineral is a naturally occurring, inorganic, homogeneous substance with a definite chemical composition and a regular three dimensional arrangement of atoms. A rock is an aggregate of minerals. From time immemorial several minerals and rocks have been used as gemstones. Nowadays not only minerals and rocks, any material, say metallic, amorphous, synthetic, organic or inorganic, that has substantial beauty, rarity and durability and can be fashioned into a desirable and attractive form is being used as a gem.
The table given below shows the gem varieties of some of the minerals/rocks commonly used as gemstones. The photos of the gemstones in the table below have been captured in the Gem Testing Lab of the Department.

NameName of mineral Chemical composition Hardness Gem Varieties
(Indian Names, if any)
Andalusite Al2SiO5 7 - 7.5 1) Andalusite (brown, yellow)
2) Viridine (green)
3) Chiastolite (black cross-like core in adark brown background)
Apatite Ca5(PO4)3 (F, Cl, O, OH)3 5 1) Apatite (white, rose, orange, yellow, red, pink, blue, green)
2 ) Apatite cat's eye
Azurite Cu3(CO3)2(OH)2 3.5 - 4 Azurite (light blue to dark blue)
Beryl Be3Al2Si6O18 7.5 - 8 1) Emerald (green)
Marathakam / Panna
2) Aquamarine (blue)
3) Heliodor (golden, yellow)
4) Morganite (pink)
5) Bixbite (red)
6) Maxixe Beryl (deep blue [fading])
7) Goshenite (colourless)
8) Beryl cat's eye
9) Star Beryl
10) Parti-coloured Beryl
11) Trapiche emerald (green with black intersecting spokes)
Calcite CaCO3 3 1) Calcite (white, yellow, brown, pink, blue)
2) Iceland spar (colourless)
Charoite(rock)Aggregate of minerals like charoite, aegirineaugite, feldspar and tinaksite .. 5 - 6 Charoite (purple with spots of black, white, grey, brown, orange)
Chrysoberyl BeAl2O4 8.5 1) Chrysoberyl (green, yellow, brown)
2) Chrysoberyl cat's eye (green, yellow, brown)
Vaiduryam/ Lasuniya/
3)Alexandrite(green in daylight and red in incandescent light)
4) Alexandrite cat's eye
Chrysocolla Cu2H2Si2O5(OH)4 2 - 4 Chrysocolla (bluish green, brownishblack)
Corundum Al2O3 9 1) Ruby (red, purplish red)
Manikyam / Manak
2) Sapphire (all colours- white, yellow, blue, green, brown)
Indraneelam / Neelam (blue)
/Manjapushyaragam / Pushkraj (yellow)
3) Padparadscha(pinkish orange)
4) Star Ruby / Sapphire
5) Colour changing Sapphire
6) Parti-coloured Sapphire
7) Trapiche Ruby / Sapphire (with six intersecting spokes)
Diamond C 10 1) Diamond (all colours)
Vajram / Heera
2) Chameleon Diamond ( yellow in dark and green in daylight)
3) Carbonado/Bort (cryptocrystalline)
Diopside MgCaSi2O6 5.5 - 6.5 1) Diopside ( yellow, black, brown, colourless)
Star Diopside
Diopside cat's eye
2) Chrome Diopside ( bright green)
3) Violane (violet blue)
Feldspar Orthoclase KAlSi3O8 6 -6.5 Moonstone (white, yellow, pink)
Moonstone cat's eye
Star Moonstone
Microcline KAlSi3O8 6 -6.5 Amazonite (green, blue, white, brownish red)
Plagioclase (Na, Ca)Al(Al, Si)Si2O8 6 -6.5 1) Labradorite (white/ grey/ yellow with play of colour)
Labradorite cat's eye
Star Labradorite
2) Sunstone
Sunstone cat's eye
Star Sunstone
Fluorite CaF2 4 1) Fluorite (all colours)
2) Blue John ( massive with blue or purple bands)
Garnet Pyrope Mg3Al2Si3O12 7 - 7.5 Pyrope (red, colourless, orange red)
Star Pyrope
Almandine Fe3Al2Si3O12 7 -7.5 Almandine ( red, purplish red, brownish black)
Star Almandine
Spessartine Mn3Al2Si3O12 7 -7.5 Spessartine ( orange, reddish orange, yellowish brown)
Spessartine cat's eye
Grossular Ca3Al2Si3O12 6.5 - 7 1) Hessonite (orange brown, orange yellow)
Gomedakam / Gomed
2) Tsavorite (green)
3) Hydrogrossular (green, pink, white)
Andradite Ca3Fe2Si3O12 6.5 - 7 1) Demantoid (green)
2) Topazolite (yellow)
3) Melanite (black and opaque)
Uvarovite Ca3Cr2Si3O12 6.5 - 7 Uvarovite (green, yellow)
7.25 Pyrope-Almandine (purplish red) (Rhodolite)
Pyrope- Spessartine ,, 7.25 Pyrope-Spessartine (bluish green in daylight and purplish red in incandescent light)
Grossular-Andradite ,, .. Grossular-Andradite (green, yellow, brown)(Mali Garnet / Grandite)
Haematite Fe2O3 5 - 6 Haematite (steel-grey to iron black)
Gun Metal
Iolite (Cordierite) (Mg,Fe3+)2Al4Si5 O18 7 -7.5 1) Iolite (blue, purple)
Star Iolite
Iolite cat's eye
2) Blood Shot Iolite ( Iolite with red hematite inclusions)
Jade Jadeite NaAlSi2O6 6.5 -7 Jadeite Jade ( white, green, lilac, pink, yellow, black, brown) Imperial Jade (emerald green)
Markal, Chashak
Nephrite Ca2 (Mg, Fe)5Si8O22(OH)2- 5 -6 Nephrite Jade (green, brown, grey, black, yellow)
Nephrite cat's eye
Kornerupine Mg3Al6 (Si, Al, B)5O21(OH) 6 - 7 Kornerupine (green, brownish yellow)
Star Kornerupine
Kornerupine cat's eye
Kyanite Al2SiO5 4 -7.5 Kyanite ( blue, green, pink, yellow, black, colourless)
Kyanite cat's eye
Lapis Lazuli(rock)Aggregate of minerals like Hauynite, Noselite, Lazurite .. 5.5 Lapis Lazuli (blue, greenish blue)
Malachite Cu2CO3 (OH)2 3.5 - 4 Malachite ( green, yellowish green, bluish green)
Dana firang
Olivine Mg2SiO4 -Fe2SiO4 6.5 - 7 Peridot (olive green, green)
Peridot cat's eye
Opal SiO2.nH2O 5.5 -6.5 1) White Opal (with play of colour)
2) Fire Opal (orange, with/without play of colour)
3) Black Opal (with play of colour)
4) Water Opal (colourless stone with play of colour)
5) Potch Opal (white with no play of colour)
6) Boulder Opal
7) Matrix Opal
8) Opal cat's eye
Pyrite FeO2 6 - 6.5 Pyrite (brassy yellow, yellow)
Quartz SiO2 7 1) Rock crystal (colourless)
2) Amethyst (purple, violet)
3) Citrine (yellow)
4) Smoky Quartz (brown, grey)
5) Rose Quartz (pink)
6) Praseolite (green)
7) Iris Quartz (colourless with iridescent cracks)
8) Aventurine Quartz (green, blue)
9) Ametrine (parti-coloured quartz-yellow& purple)
10) Sagenitic Quartz (colourless withtourmaline/rutile needles)
11) Quartz cat's eye
12) Star Quartz
13) Tiger's eye (golden brown)
14) Hawk's eye (blue)
(Cryptocrystalline Quartz)
6.5 1) Milky Chalcedony (white)
2) Carnelian (red)
3) Chrysoprase (apple green)
4) Prase (yellow green)
5) Blood Stone/Heliotrope (dark green with red spots)
6) Chrome Chalcedony (green)
7) Sard ( brown)
8) Plasma (green)
9) Agate ( curved bands)
10) Moss Agate (dendritic)
11) Fire Agate (iridescence)
12) Onyx (straight bands)
13) Sardonyx (brown)
14) Jasper (red, yellow, black)
Rhodochrosite MnCO3 4 Rhodochrosite-
banded (pink, grey, brown)-
transparent (pink, red)
Rhodonite MnSiO3 6 Rhodonite (pinkish red, orange red, brownish red; massive- pink with black veins)
Scapolite (Na,Ca)4Al3(Al,Si)3Si6O24(Cl,F,OH, CO3, SO4) 6 Scapolite (all colours)
Star Scapolite
Sillimanite Al2SiO5 6.5 -7.5 Sillimanite (all colours)
Sillimanite cat's eye (all colours)
Star Sillimanite
Sphene CaTiSiO5 5 - 5.5 Sphene (yellow, green, brown,colourless)
Star Sphene
Spinel MgAl2O4 7.5 - 8 1) Spinel (all colours)
2) Star Spinel
3) Colour changing Spinel
Sapphirine Mg3.5Al9.0Si1.5O20 7.5 Sapphirine (blue, green, greenish grey)
Spodumene LiAlSi2O6 6.5 -7.5 1) Spodumene (colourless, blue, yellow, yellowish green)
2) Kunzite (pink, purple)
3) Hiddenite (green, bluish green)
Staurolite Fe2Al9Si4O22(OH)2 7 - 7.5 Staurolite (brown, yellowish brown)
Taaffeite BeMgAl4O8 8 - 8.5 1) Taaffeite (colourless, purple, red, blue)
2) Colour changing Taaffeite (bluish in daylight and reddish in incandescent light)
Topaz Al2SiO4(F,OH)2 8 Topaz (Colourless, blue, orange, pink, red, brown, yellow)
Topaz cat's eye
Tourmaline (Na, Ca) (Li, Mg, Fe, Al)9 B3Si6(O, OH)31 7- 7.5 1) Tourmaline (all colours)
2) Parti-coloured Tourmaline (watermelon tourmaline)
3) Tourmaline cat's eye
4) Colour changing Tourmaline
Turquoise CuAl6(PO4)4(OH)8.4-5H2O 5 - 6 Turquoise (blue to green, yellowish green,greenish blue)
Zircon ZrSiO4 7 - 7.5 Zircon (all colours)
Zircon cat's eye
Zoisite Ca2Al3Si3O12OH 6.5 - 7 1) Tanzanite (blue)
2) Thulite (pink)
3) Anyolite (green with translucent rubies)
4) Zoisite (yellow, brown, violet)
5) Zoisite cat's eye

Organic Gem
An organic gem is one that was made by living things, present or past. Examples include pearl, coral, jet, ivory, shell, amber, ammolite etc. Such gems consist of the molecules formed by the organism, although these molecules may have been altered somewhat due to compression or other geological or chemical forces.
Synthetics and Simulants
Several of these gemstones are synthesized for commercial purposes. Synthetic gemstones correspond to their naturally occurring counterparts in crystal structure, physical, chemical and optical properties. Usually natural stones are distinguished from synthetic stones with the help of inclusions present.
Simulants are imitation gems that look like the natural or synthetic gems they imitate, but do not have the physical, chemical and optical properties of the gems they mimic. Two common gem simulants are glass and plastic.

Methods used for synthesis of gemstones Gemstones synthesized
Flame fusion (Verneuil) Synthetic Ruby/Sapphire
Synthetic Spinel
Flux Fusion Synthetic Emerald
Synthetic Ruby/Sapphire
Synthetic Spinel
Synthetic Chrysoberyl cat's eye
Synthetic Alexandrite
Synthetic Alexandrite cat's eye
Synthetic Diamond
Hydrothermal Synthetic Emerald
Synthetic Beryl
Synthetic Ruby/Sapphire
Synthetic Quartz
Czochralski Pulling Synthetic Ruby/Sapphire
Synthetic Alexandrite
Gadolinium Gallium Garnet
Yttrium Aluminium Garnet
Skull Melting Synthetic Cubic Zirconia
Carbon Vapour Deposition Method Synthetic Diamond
Gel Growth Process Synthetic Opal
Ceramic Techniques Lapis Lazuli

Several treatments are done on gemstones to improve their colour, clarity and durability, thereby enhancing the general appearance and beauty of the stones. The common treatments done are:

Sl. No. Treatments Gemstones usually treated
1 Bleaching Pearl, Coral, Jade, Tiger's eye,Ivory
2 Coating and foiling Topaz, Quartz, Ammolite, Jade
3 Fracture Filling with oil, wax, resins, polymers, glass Emerald , Corundum, Aquamarine, Iolite, Amethyst, Peridot, Tourmaline, Diamond, Jade, Opal
4 Dyeing (Staining) Lapis Lazuli, Chalcedony, Corundum, Emerald, Opal, Jade
5 Irradiation Beryl, Quartz, Sapphire, Diamond, Topaz, Tourmaline, Kunzite, Pearl, Chrysoberyl
6 Heating Ruby, Sapphire, Chalcedony, Beryl, Diamond, Topaz, Tourmaline, Zoisite, Zircon, Quartz, Jade, Amber
7 Diffusion Ruby, Sapphire
8 High pressure high temperature (HPHT) Diamond
9 Laser drilling Diamond