Refractive Index Testing
Overview
The refractive index (RI) is a fundamental optical property that measures how much light bends when entering a gemstone. It is one of the most important diagnostic tools in gemology.
How It Works
A refractometer measures the critical angle of total internal reflection. Light enters the gem and bends at an angle determined by the gem's optical density. The RI reading helps identify gem species.
Equipment Needed
- Gemological refractometer
- Refractive index liquid (RI 1.81)
- Polarizing filter
- Light source (sodium or white LED)
Procedure
- Clean the gem and refractometer hemicylinder
- Apply a small drop of RI liquid to the hemicylinder
- Place the gem flat on the liquid with the largest facet down
- Look through the eyepiece and read where the shadow edge meets the scale
- Rotate the gem to check for double refraction (birefringence)
Typical Values / Results
| Gemstone | Value / Observation |
|---|---|
| Diamond | 2.4170-2.4190 |
| Alexandrite | 1.7460-1.7550 |
| Emerald | 1.5770-1.5830 |
| Paraiba Tourmaline | 1.6240-1.6440 |
| Musgravite | 1.7180-1.7240 |
| Painite | 1.7870-1.8160 |
| Taaffeite | 1.7190-1.7230 |
| Ruby | 1.7620-1.7700 |
| Red Spinel | 1.7120-1.7360 |
| Bixbite | 1.5770-1.5830 |
| Grandidierite | 1.5900-1.6230 |
| Sapphire | 1.7620-1.7700 |
| Tanzanite | 1.6910-1.7050 |
| Blue Spinel | 1.7120-1.7360 |
| Imperial Topaz | 1.6190-1.6270 |
| Serendibite | 1.7010-1.7060 |
| Vayrynenite | 1.6400-1.6700 |
| Benitoite | 1.7570-1.8040 |
| Demantoid | 1.8800-1.9400 |
| Tsavorite | 1.7400-1.7600 |
| Padparadscha Sapphire | 1.7620-1.7700 |
| Clinohumite | 1.6290-1.6740 |
| Afghanite | 1.5230-1.5290 |
| Jeremejevite | 1.6370-1.6530 |
| Pink Spinel | 1.7120-1.7360 |
| Pink Topaz | 1.6190-1.6270 |
| Sphene | 1.8850-2.0500 |
| Rubellite | 1.6240-1.6440 |
| Indicolite | 1.6240-1.6440 |
| Sphalerite | 2.3700-2.4300 |
| Hiddenite | 1.6600-1.6810 |
| Diaspore | 1.7020-1.7500 |
| Andradite | 1.8800-1.8900 |
| Watermelon Tourmaline | 1.6240-1.6440 |
| Andalusite | 1.6290-1.6500 |
| Kunzite | 1.6600-1.6810 |
| Aquamarine | 1.5770-1.5830 |
| Morganite | 1.5770-1.5830 |
| Heliodor | 1.5770-1.5830 |
| Goshenite | 1.5770-1.5830 |
| Green Beryl | 1.5770-1.5830 |
| Cat's Eye Chrysoberyl | 1.7460-1.7550 |
| Star Sapphire | 1.7620-1.7700 |
| Star Ruby | 1.7620-1.7700 |
| Moonstone | 1.5180-1.5260 |
| Labradorite | 1.5590-1.5680 |
| Sunstone | 1.5250-1.5480 |
| Amazonite | 1.5220-1.5300 |
| Orthoclase | 1.5180-1.5260 |
| Andesine | 1.5430-1.5510 |
| Oligoclase | 1.5430-1.5510 |
| Black Opal | 1.3700-1.4700 |
| Fire Opal | 1.3700-1.4500 |
| Crystal Opal | 1.3700-1.4700 |
| Boulder Opal | 1.3700-1.4700 |
| White Opal | 1.3700-1.4700 |
| Jelly Opal | 1.3700-1.4700 |
| Hyalite | 1.4000-1.4600 |
| Peruvian Opal | 1.3700-1.4700 |
| Amethyst | 1.5440-1.5530 |
| Citrine | 1.5440-1.5530 |
| Rose Quartz | 1.5440-1.5530 |
| Smoky Quartz | 1.5440-1.5530 |
| Ametrine | 1.5440-1.5530 |
| Prasiolite | 1.5440-1.5530 |
| Milky Quartz | 1.5440-1.5530 |
| Rock Crystal | 1.5440-1.5530 |
| Rutilated Quartz | 1.5440-1.5530 |
| Tourmalinated Quartz | 1.5440-1.5530 |
| Aventurine | 1.5440-1.5530 |
| Tiger's Eye | 1.5440-1.5530 |
| Hawk's Eye | 1.5440-1.5530 |
| Chalcedony | 1.5300-1.5390 |
| Agate | 1.5300-1.5390 |
| Carnelian | 1.5300-1.5390 |
| Chrysoprase | 1.5300-1.5390 |
| Onyx | 1.5300-1.5390 |
| Sardonyx | 1.5300-1.5390 |
| Bloodstone | 1.5300-1.5390 |
| Jasper | 1.5400-1.5500 |
| Peridot | 1.6540-1.6900 |
| Zircon | 1.9250-2.0150 |
| Rhodolite | 1.7400-1.7700 |
| Almandine | 1.7800-1.8100 |
| Pyrope | 1.7300-1.7600 |
| Spessartite | 1.7900-1.8100 |
| Grossular | 1.7300-1.7600 |
| Uvarovite | 1.7400-1.8700 |
| Mali Garnet | 1.7400-1.7600 |
| Color-Change Garnet | 1.7400-1.8700 |
| Rhodochrosite | 1.7110-1.7520 |
| Iolite | 1.5420-1.5510 |
| Kyanite | 1.7100-1.7340 |
| Kornerupine | 1.6670-1.7000 |
| Seraphinite | 1.5710-1.5990 |
| Charoite | 1.5500-1.5590 |
| Sugilite | 1.6070-1.6100 |
| Larimar | 1.5950-1.6450 |
| Jadeite | 1.6600-1.6800 |
| Nephrite | 1.6060-1.6320 |
| Lapis Lazuli | 1.5000-1.5500 |
| Turquoise | 1.6100-1.6500 |
| Malachite | 1.6550-1.9090 |
| Azurite | 1.7300-1.8380 |
| Rhodonite | 1.7110-1.7520 |
| Sodalite | 1.4830-1.4870 |
| Prehnite | 1.6110-1.6650 |
| Fluorite | 1.4340-1.4340 |
| Apatite | 1.6340-1.6380 |
| Scapolite | 1.5400-1.5790 |
| Diopside | 1.6640-1.7210 |
| Enstatite | 1.6500-1.6800 |
| Hypersthene | 1.6730-1.7310 |
| Burmese Spinel | 1.7120-1.7360 |
| Tanzanian Spinel | 1.7120-1.7360 |
| Vietnamese Purple Spinel | 1.7120-1.7360 |
| Quartz | 1.5440-1.5530 |
| Serpentine | 1.4900-1.5700 |
| Aragonite | 1.5300-1.6860 |
| Black Diamond | 2.4170-2.4190 |
| Phenakite | 1.6540-1.7010 |
| Verdelite | 1.6240-1.6440 |
| Tourmaline | 1.6240-1.6440 |
| Topaz | 1.6190-1.6270 |
| Opal | 1.3700-1.4700 |
| Spinel | 1.7120-1.7360 |
| Garnet | 1.7200-1.9400 |
| Pink Diamond | 2.4170-2.4190 |
| Yellow Diamond | 2.4170-2.4190 |
| Brown Diamond | 2.4170-2.4190 |
| Chrysoberyl | 1.7460-1.7550 |
| Danburite | 1.6300-1.6360 |
| Tantalite | 2.2600-2.4300 |
| Bastnasite | 1.7170-1.8180 |
| Xenotime | 1.7210-1.8270 |
| Erythrite | 1.6260-1.7010 |
| Scheelite | 1.9180-1.9370 |
| Cassiterite | 1.9970-2.0930 |
| Hackmanite | 1.4830-1.4870 |
| Hauyne | 1.4960-1.5050 |
| Poudretteite | 1.5100-1.5320 |
| Spodumene | 1.6600-1.6810 |
| Malaya Garnet | 1.7400-1.7700 |
| Mahenge Spinel | 1.7120-1.7360 |
| Pezzottaite | 1.6010-1.6200 |
| Green Diamond | 2.4170-2.4190 |
| Blue Diamond | 2.4170-2.4190 |
| Chrome Sphene | 1.8850-2.0500 |
| Blue Topaz | 1.6190-1.6270 |
| Feldspar | 1.5180-1.5880 |
| Opaque Emerald | 1.5770-1.5830 |
| Opaque Hauyne | 1.4960-1.5050 |
| Hematite | 2.9400-3.2200 |
| Cuprite | 2.8490-2.8490 |
| Cerussite | 1.8040-2.0770 |
| Calcite | 1.4860-1.6580 |
| Smithsonite | 1.6210-1.8490 |
| Brazilianite | 1.6020-1.6210 |
| Amblygonite | 1.5780-1.6060 |
| Bytownite | 1.5630-1.5720 |
| Pollucite | 1.5170-1.5250 |
| Petalite | 1.5040-1.5190 |
| Phosphophyllite | 1.5940-1.6210 |
| Dioptase | 1.6440-1.7090 |
| Achroite | 1.6240-1.6440 |
| Albite | 1.5250-1.5480 |
| Elbaite | 1.6240-1.6440 |
| Axinite | 1.6740-1.7040 |
| Vesuvianite | 1.7030-1.7520 |
| Epidote | 1.7290-1.7680 |
| Zoisite | 1.6910-1.7050 |
| Sillimanite | 1.6570-1.6840 |
| Euclase | 1.6520-1.6720 |
| Moldavite | 1.4800-1.5400 |
| Beryl | 1.5770-1.5830 |
| Triplite | 1.6800-1.7500 |
| Sapphirine | 1.7010-1.7190 |
| Pargasite | 1.6120-1.6460 |
| Hambergite | 1.5470-1.5630 |
| Parisite | 1.6720-1.7710 |
| Idocrase | 1.7030-1.7520 |
| Herderite | 1.5940-1.6130 |
| Chondrodite | 1.5920-1.6370 |
| Stibiotantalite | 2.3740-2.4510 |
| Chrome Tourmaline | 1.6240-1.6440 |
| Rhodizite | 1.6890-1.6940 |
| Hessonite | 1.7400-1.7600 |
| Color Change Garnet | 1.7400-1.8700 |
| Color Change Sapphire | 1.7620-1.7700 |
| Hydrogrossular | 1.7200-1.7350 |
| Ammolite | 1.5200-1.6800 |
| Amber | 1.5400-1.5400 |
| Lepidolite | 1.5250-1.5880 |
| Brucite | 1.5600-1.5800 |
| Mawsitsit | 1.5200-1.6740 |
| Hemimorphite | 1.6140-1.6360 |
| Fire Agate | 1.5300-1.5390 |
| Synthetic Sapphire | 1.7620-1.7700 |
| Synthetic Ruby | 1.7620-1.7700 |
| Synthetic Emerald | 1.5770-1.5830 |
| Synthetic Moissanite | 2.6480-2.6910 |
Limitations
Cannot read gems with RI above 1.81 (the liquid limit). Curved surfaces and small stones are difficult to measure accurately.
Gemstones That Cannot Be Tested
The following gemstones cannot be fully tested with this method:
Exceptional Brilliance (RI > 2.4)
- Diamond (RI: 2.4170-2.4190)
- Black Diamond (RI: 2.4170-2.4190)
- Pink Diamond (RI: 2.4170-2.4190)
- Yellow Diamond (RI: 2.4170-2.4190)
- Brown Diamond (RI: 2.4170-2.4190)
- Green Diamond (RI: 2.4170-2.4190)
- Blue Diamond (RI: 2.4170-2.4190)
- Hematite (RI: 2.9400-3.2200)
- Cuprite (RI: 2.8490-2.8490)
- Synthetic Moissanite (RI: 2.6480-2.6910)
Partial Range Issues (upper range exceeds 1.81)
- Painite (RI: 1.7870-1.8160)
- Uvarovite (RI: 1.7400-1.8700)
- Color-Change Garnet (RI: 1.7400-1.8700)
- Malachite (RI: 1.6550-1.9090)
- Azurite (RI: 1.7300-1.8380)
- Garnet (RI: 1.7200-1.9400)
- Bastnasite (RI: 1.7170-1.8180)
- Xenotime (RI: 1.7210-1.8270)
- Cerussite (RI: 1.8040-2.0770)
- Smithsonite (RI: 1.6210-1.8490)
- Color Change Garnet (RI: 1.7400-1.8700)
Excellent Brilliance (RI > 1.81)
- Demantoid (RI: 1.8800-1.9400)
- Sphene (RI: 1.8850-2.0500)
- Andradite (RI: 1.8800-1.8900)
- Chrome Sphene (RI: 1.8850-2.0500)
Outstanding Brilliance (RI 1.9-2.4)
- Sphalerite (RI: 2.3700-2.4300)
- Zircon (RI: 1.9250-2.0150)
- Tantalite (RI: 2.2600-2.4300)
- Scheelite (RI: 1.9180-1.9370)
- Cassiterite (RI: 1.9970-2.0930)
- Stibiotantalite (RI: 2.3740-2.4510)
For these gemstones, alternative testing methods such as specific gravity, spectroscopy, or professional lab equipment with extended RI ranges must be used.