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Guide Gemstone Refractive Index

 In gemology, refractive index is one of the chief means of identifying a gemstone. It is measured using an instrument known as a refractometer.

The refractive index (often abbreviated RI) is an optical property that refers to the ratio of the speed of light in a vacuum to the speed of light through a transparent medium. Light is bent as it moves from air to another material, an effect familiar from observing how a stick partially immersed in water appears to be bent.

Gems with a higher refractive index tend to show more brilliance, since light is bent at a higher angle when it passes into the material. More light is then reflected back out the crown to the eye, rather than passing straight through the gem.

Most gemstones have 2 refractive indices, since light is split into 2 beams travelling at different speeds when it enters the gem. This phenomenon is known as birefringence or double refraction, and it is expressed as a value that indicates the difference between the two RI's for the gem material.

Some gemstones are singly refractive and have only 1 RI. Singly refractive gems include diamond, spinel and all the garnets, as well as other cubic crystals like cuprite, fluorite and sphalerite. Amorphous gems -- those with no crystal structure -- are also singly refractive.

 

 

 Gem
Variety		 Refractive
Index 		Double
Refraction
Hematite Hematite 2.940-3.220 0.287
Cinnabar Cinnabar 2.905-3.256 0.351
Cuprite Cuprite 2.849 none
Natural Diamond Diamond 2.417-2.419 none
Sphalerite Sphalerite 2.368-2.371 none 
Cassiterite Cassiterite 1.997-2.098  0.096-0.098
Scheelite Scheelite 1.918-1.937  0.010-0.018
Demantoid Garnet Demantoid
Garnet		 1.88-1.94  none
Uvarovite Garnet Uvarovite
Garnet		 1.865 none
Sphene (Titanite) Sphene 
(Titanite)		 1.843-2.110 0.100-0.192
Zircon Zircon 1.810-2.024 0.002-0.059
Cerussite Cerussite 1.804-2.079 0.274
Spessartite Garnet Spessartite
Garnet		 1.790-1.820 none
Painite Painite 1.787-1.816 0.029
Almandine Garnet Almandine 
Garnet		 1.770-1.820 none
Natural Ruby Ruby 1.762-1.778 0.008 
Star Ruby Star Ruby 1.762-1.778 0.008 
Sapphire Sapphire 1.762-1.778 0.008
Star Sapphire Star
Sapphire		 1.762-1.778 0.008 
Mali Garnet Mali Garnet 1.760-1.780 none
Benitoite Benitoite 1.757-1.804 0.047
Color Change Alexandrite Alexandrite 1.746-1.763 0.007-0.011
Alexandite Cat's Eye Alexandrite
Cat's Eye		 1.746-1.763 0.007-0.011
Chrysoberyl Chrysoberyl 1.746-1.763 0.007-0.011 
Chrysoberyl Cat's Eye Chrysoberyl
Cat's Eye		 1.746-1.763 0.007-0.011 
Vanadium Chrysoberyl Vanadium
Chrysoberyl		 1.746-1.763 0.007-0.011 
Rhodolite Garnet Rhodolite
Garnet		 1.742-1.785 none
Color Change Garnet Color-Change 
Garnet		 1.742-1.780 none
Malaia Garnet Malaia Garnet 1.742-1.780 none
Grossular Garnet Grossular
Garnet		 1.734-1.759 none
Tsavorite Garnet Tsavorite
Garnet		 1.734-1.759 none
Leuco Garnet Leuco Garnet 1.734-1.759 none
Hessonite Garnet Hessonite
Garnet		 1.730-1.757 none
Epidote Epidote 1.729-1.768 0.015-0.049
Bastnasite Bastnasite 1.720-1.820 0.100
Azurite Azurite 1.720-1.848 0.108-0.110
Pyrope Garnet Pyrope
Garnet		 1.720-1.756  none
Taaffeite Taaffeite 1.719-1.730 0.004-0.009
Rhodonite Rhodonite 1.716-1.752 0.010-0.014
Spinel Spinel 1.712-1.782  none
Kyanite Kyanite 1.710-1.734 0.015-0.033
Diaspore Diaspore 1.702-1.750 0.048
Serendibite Serendibite 1.701-1.743 0.005
Idocrase (Vesuvianite) Idocrase 
(Vesuvianite)		 1.700-1.723  0.002-0.012
Tanzanite Tanzanite 1.691-1.700 0.009
Zoisite Zoisite 1.691-1.700 0.009
Dumortierite Dumortierite 1.678-1.689 0.015-0.037
Sinhalite Sinhalite 1.665-1.712 0.036-0.042
Chrome Diopside Chrome
Diopside		 1.664-1.730 0.024-0.031
Diopside Diopside 1.664-1.730 0.024-0.031
Kornerupine Kornerupine 1.660-1.699 0.012-0.017
Hiddenite Hiddenite 1.660-1.681 0.014-0.016
Kunzite Kunzite 1.660-1.681 0.014-0.016
Spodumene Spodumene 1.660-1.681 0.014-0.016
Axinite Axinite 1.656-1.704 0.010-0.012
Malachite Malachite 1.666-1.909 0.254
Sillimanite Sillimanite 1.655-1.684 0.014-0.021
Jadeite Jade Jadeite Jade 1.652-1.688 0.020
Peridot Peridot 1.650-1.703 0.036-0.038
Enstatite Enstatite 1.650-1.680 0.009-0.012
Euclase Euclase 1.650-1.677 0.019-0.025
Phenakite Phenakite 1.650-1.670 0.016
Dioptase Dioptase 1.644-1.709 0.051-0.053
Jet Jet 1.640-1.680 none
Jeremejevite Jeremejevite 1.637-1.653 0.007-0.013
Barite Barite 1.636-1.648 0.012
Danburite Danburite 1.630-1.636 0.006-0.008
Clinohumite Clinohumite 1.629-1.674 0.028-0.041
Apatite Apatite 1.628-1.649 0.002-0.006
Cat's Eye Apatite Cat's Eye
Apatite		 1.628-1.649 0.002-0.006
Andalusite Andalusite 1.627-1.649 0.007-0.013
Smithsonite Smithsonite 1.621-1.849 0.228
Datolite Datolite 1.621-1.675 0.040-0.060
Celestine Celestine 1.619-1.635 0.010-0.012
Chrome Tourmaline Chrome 
Tourmaline		 1.614-1.666 0.014-0.032
Paraiba Tourmaline Paraiba 
Tourmaline		 1.614-1.666 0.014-0.032
Rubellite Tourmaline Rubellite 
Tourmaline		 1.614-1.666 0.014-0.032
Tourmaline Tourmaline 1.614-1.666 0.014-0.032
Actinolite Actinolite 1.614-1.653 0.020-0.025
Hemimorphite Hemimorphite 1.614-1.638 0.022
Prehnite Prehnite 1.611-1.669 0.021-0.039
Gaspeite Gaspeite 1.610-1.810 0.22
Turquoise Turquoise 1.610-1.650 0.040
Topaz Topaz 1.609-1.643  0.009-0.016
Imperial Topaz Imperial
Topaz		 1.609-1.643  0.009-0.016 
Sugilite Sugilite 1.607-1.611 0.001-0.004
Brazilianite Brazilianite 1.602-1.623 0.019-0.021
Pezzotaite Pezzotaite 1.601-1.620 0.008-0.011
Rhodochrosite Rhodochrosite 1.600-1.820 0.208-0.220
Nephrite Jade Nephrite Jade 1.600-1.627 0.027
Larimar Larimar 1.595-1.645 0.038
Eudialyte Eudialyte 1.591-1.633 0.003-0.010
Grandidierite Grandidierite 1,590-1.623 0.033
Howlite Howlite 1.586-1.605 0.019
Amblygonite Amblygonite 1.578-1.646 0.024-0.030
Emerald Emerald 1.565-1.602 0.005
Aquamarine Aquamarine 1.565-1.602 0.005
Morganite Morganite 1.562-1.605  0.004-0.010
Goshenite Beryl Goshenite
Beryl		 1.562-1.605 0.004-0.010 
Golden Beryl Golden Beryl 1.562-1.605 0.004-0.010  
Red Beryl Red Beryl
(Bixbite)		 1.562-1.605 0.004-0.010 
Labradorite Labradorite 1.559-1.570 0.008-0.010
Hambergite Hambergite 1.553-1.628  0.072
Charoite Charoite 1.550-1.561 0.004-0.009
Amethyst Amethyst 1.544-1.553 0.009 
Aventurine Aventurine 1.544-1.553  0.009
Ametrine Ametrine 1.544-1.553 0.009 
Citrine Citrine 1.544-1.553 0.009 
Gem Silica Gem Silica 1.544-1.553 0.009 
Rock Crystal Rock Crystal 1.544-1.553 0.009 
Rose Quartz Rose
Quartz		 1.544-1.553 0.009 
Smoky Quartz Smoky
Quartz		 1.544-1.553 0.009 
Andesine Andesine 1.543-1.551 0.009
Iolite Iolite 1.542-1.578 0.008-0.012
Scapolite Scapolite 1.540-1.579 0.009-0.037
Jasper Jasper 1.54 none
Amber Amber 1.539-1.545 none
Oligoclase Oligoclase 1.538-1.550 0.007-0.013
Aragonite Aragonite 1.530-1.685 0.155
Williamsite Williamsite 1.530-1.575 0.001-0.002
Agate Agate 1.530-1.540 0.004-0.009
Bloodstone Bloodstone 1.530-1.540 0.004-0.009
Chalcedony Chalcedony 1.530-1.540 0.004-0.009
Chrome Chalcedony Chrome 
Chalcedony		 1.530-1.540 0.004-0.009
Carnelian Carnelian 1.530-1.540 0.004-0.009
Chrysoprase Chrysoprase 1.530-1.540 0.004-0.009
Onyx Onyx 1.530-1.540 0.004-0.009
Lepidolite Lepidolite 1.525-1.587 0.013
Sunstone Sunstone 1.525-1.548 0.010
Amazonite Amazonite 1.522-1.530 0.008
Pearl Pearl 1.52-1.69 0.158
Ammolite Ammolite
(Korite)		 1.52-1.68 0.155
Orthoclase Orthoclase 1.518-1.530 0.008
Moonstone Moonstone 1.518-1.528 0.008
Carletonite Carletonite 1.517-1.521 0.004
Poudretteite Poudretteite 1.511-1.532 0.021
Petalite Petalite 1.502-1.518 0.012-0.017
Lapis Lazuli Lapis Lazuli 1.50 none
Hauyne Hauyne 1.498-1.510 none
Ulexite Ulexite 1.491-1.520 0.029
Calcite Calcite 1.486-1.658 0.172
Cobaltoalcite Cobaltoalcite 1.486-1.658 0.172
Coral Coral 1.486-1.658 0.160-0.172
Moldavite Moldavite 1.48-1.54 none
Hackmanite Hackmanite 1.48 none
Sodalite Sodalite 1.48 none
Chrysocolla Chrysocolla 1.46-1.57 0.023-0.040
Obsidian Obsidian 1.45-1.55 none
Fluorite Fluorite 1.434 none
Mexican Fire Opal Mexican 
Fire Opal		 1.37-1.52 none
Opal Opal 1.37-1.52 none