Rubies and spinels may show similar colors. One of the simplest ways to distinguish these gems is to test for double refraction with a dichroscope. Since spinels have an isometric crystal structure, they have no double refraction. On the other hand, rubies do have double refraction. Brilliant-cut oval spinel, 6.88 cts, recut by AGTA Spectrum Award-winner Mark Kaufman. Mogok gem tract, Myanmar. © Rob Lavinsky, www.iRocks.com. Used with permission.
Refractive Indices and Double Refraction Values
The following table organizes gemstones from highest to lowest refractive indices.
Gemstone | Refractive Index | Double Refraction |
---|---|---|
Hematite | 2.940-3.220 | 0.287 |
Cinnabar | 2.905-3.256 | 0.351 |
Proustite | 2.881-3.084 | 0.203 |
Pyrargyrite | 2.880-3.080 | 0.200 |
Cuprite | 2.849 | none |
Rutile | 2.616-2.903 | 0.287 |
Brookite | 2.583-2.700 | 0.117 |
Anatase | 2.488-2.564 | 0.046-0.067 |
Diamond | 2.417-2.419 | anomalous |
Fabulite | 2.409 | none |
Stibiotantalite | 2.370-2.450 | 0.080 |
Sphalerite | 2.368-2.371 | none |
Crocoite | 2.290-2.660 | 0.270 |
Wulfenite | 2.280-2.400 | 0.120 |
Tantalite | 2.260-2.430 | 0.160 |
Linobate | 2.210-2.300 | 0.090 |
Manganotantalite | 2.190-2.340 | 0.150 |
Cubic zirconia (CZ) | 2.150-2.180 | none |
Mimetite | 2.120-2.135 | 0.015 |
Phosgenite | 2.114-2.145 | 0.028 |
Senarmontite | 2.087 | none |
Boleite | 2.030-2.050 | 0.020 |
Zincite | 2.013-2.029 | 0.016 |
Cassiterite | 1.997-2.098 | 0.096-0.098 |
Simpsonite | 1.994-2.040 | 0.058 |
Gadolinium gallium garnet (GGG) | 1.970-2.020 | 0.058 |
Sulfur | 1.958-2.245 | 0.291 |
Bayldonite | 1.950-1.990 | 0.04 |
Scheelite | 1.918-1.937 | 0.010-0.018 |
Andradite garnet | 1.880-1.940 | none |
Anglesite | 1.878-1.895 | 0.017 |
Uvarovite garnet | 1.865 | none |
Purpurite | 1.850-1.920 | 0.007 |
Sphene (titanite) | 1.843-2.110 | 0.100-0.192 |
Yttrium aluminum garnet (YAG) | 1.833-1.835 | none |
Zircon | 1.810-2.024 | 0.002-0.059 |
Cerussite | 1.804-2.079 | 0.274 |
Gahnite | 1.791-1.818 | none |
Spessartite garnet | 1.790-1.820 | none |
Painite | 1.787-1.816 | 0.029 |
Monazite | 1.774-1.849 | 0.049-0.055 |
Almandine garnet | 1.770-1.820 | none |
Gadolinite | 1.770-1.820 | 0.01-0.04 |
Ruby (corundum) | 1.762-1.778 | 0.008 |
Sapphire (corundum) | 1.762-1.778 | 0.008 |
Benitoite | 1.757-1.804 | 0.047 |
Shattuckite | 1.752-1.815 | 0.063 |
Chrysoberyl | 1.746-1.763 | 0.007-0.011 |
Periclase | 1.740 | none |
Scorodite | 1.738-1.816 | 0.027-0.030 |
Staurolite | 1.736-1.762 | 0.010-0.015 |
Grossular garnet | 1.734-1.759 | none |
Chambersite | 1.732-1.744 | 0.012 |
Hessonite garnet | 1.730-1.757 | none |
Epidote | 1.729-1.768 | 0.015-0.049 |
Pyroxmangite | 1.726-1.764 | 0.016-0.020 |
Azurite | 1.720-1.848 | 0.108-0.110 |
Pyrope garnet | 1.720-1.756 | none |
Hodgkinsonite | 1.719-1.748 | 0.022-0.026 |
Taaffeite | 1.719-1.730 | 0.004-0.009 |
Rhodonite | 1.716-1.752 | 0.010-0.014 |
Gahnospinel | 1.715-1.754 | none |
Spinel | 1.712-1.762 | none |
Kyanite | 1.710-1.734 | 0.015-0.033 |
Adamite | 1.708-1.760 | 0.048-0.050 |
Diaspore | 1.702-1.750 | 0.048 |
Serendibite | 1.701-1.743 | 0.005 |
Sapphirine | 1.701-1.734 | 0.004-0.007 |
Aegirine-augite | 1.700-1.800 | 0.030-0.050 |
Idocrase (vesuvianite) | 1.700-1.723 | 0.002-0.012 |
Tanzanite | 1.691-1.700 | 0.009 |
Neptunite | 1.690-1.736 | 0.029-0.045 |
Willemite | 1.690-1.723 | 0.028-0.033 |
Rhodizite | 1.690 | none |
Triphylite | 1.689-1.702 | 0.006-0.008 |
Lithiophilite | 1.680-1.700 | 0.01 |
Dumortierite | 1.678-1.689 | 0.015-0.037 |
Legrandite | 1.675-1.740 | 0.060 |
Hypersthene | 1.673-1.731 | 0.010-0.016 |
Parisite | 1.671-1.772 | 0.081-0.101 |
Clinozoisite | 1.670-1.734 | 0.010 |
Sinhalite | 1.665-1.712 | 0.036-0.042 |
Lawsonite | 1.665-1.686 | 0.019-0.021 |
Diopside | 1.664-1.730 | 0.024-0.031 |
Bustamite | 1.662-1.707 | 0.014-0.015 |
Kornerupine | 1.660-1.699 | 0.012-0.017 |
Hiddenite | 1.660-1.681 | 0.014-0.016 |
Kunzite | 1.660-1.681 | 0.014-0.016 |
Boracite | 1.658-1.673 | 0.010-0.011 |
Axinite | 1.656-1.704 | 0.010-0.012 |
Malachite | 1.655-1.909 | 0.254 |
Sillimanite | 1.655-1.684 | 0.014-0.021 |
Jadeite | 1.652-1.688 | 0.20 |
Peridot | 1.650-1.703 | 0.036-0.038 |
Ludlamite | 1.650-1.697 | 0.038-0.044 |
Enstatite | 1.650-1.680 | 0.009-0.012 |
Euclase | 1.650-1.677 | 0.019-0.025 |
Phenakite | 1.650-1.670 | 0.016 |
Dioptase | 1.644-1.709 | 0.051-0.053 |
Jet | 1.640-1.680 | none |
Eosphorite | 1.638-1.671 | 0.028-0.035 |
Spurrite | 1.637-1.681 | 0.039-0.040 |
Jeremejevite | 1.637-1.653 | 0.007-0.013 |
Barite | 1.636-1.648 | 0.012 |
Siderite | 1.633-1.875 | 0.242 |
Danburite | 1.630-1.636 | 0.006-0.008 |
Clinohumite | 1.629-1.674 | 0.028-0.041 |
Apatite | 1.628-1.649 | 0.002-0.006 |
Andalusite | 1.627-1.649 | 0.007-0.013 |
Friedelite | 1.625-1.664 | 0.030 |
Smithsonite | 1.621-1.849 | 0.228 |
Datolite | 1.621-1.675 | 0.040-0.050 |
Celestite | 1.619-1.635 | 0.010-0.012 |
Tourmaline | 1.614-1.666 | 0.014-0.032 |
Actinolite | 1.614-1.653 | 0.020-0.025 |
Hemimorphite | 1.614-1.636 | 0.022 |
Lazulite | 1.612-1.646 | 0.031-0.036 |
Prehnite | 1.611-1.669 | 0.021-0.039 |
Gaspéite | 1.610-1.810 | 0.22 |
Turquoise | 1.610-1.650 | 0.040 |
Topaz | 1.609-1.643 | 0.008-0.016 |
Sugilite | 1.607-1.611 | 0.001-0.004 |
Sogdianite | 1.606-1.608 | 0.002 |
Brazilianite | 1.602-1.623 | 0.019-0.021 |
Rhodochrosite | 1.600-1.820 | 0.208-0.220 |
Odontolite | 1.600-1.640 | 0.010 |
Nephrite | 1.600-1.627 | 0.027 |
Pectolite (larimar) | 1.595-1.645 | 0.038 |
Montebrasite | 1.594-1.633 | 0.22 |
Phosphophyllite | 1.594-1.621 | 0.021-0.033 |
Meliphanite | 1.593-1.612 | 0.019 |
Eudialyte | 1.591-1.633 | 0.003-0.010 |
Chondrodite | 1.592-1.646 | 0.028-0.034 |
Catapleiite | 1.590-1.629 | 0.039 |
Wardite | 1.590-1.599 | 0.009 |
Herderite | 1.587-1.627 | 0.023-0.032 |
Colemanite | 1.586-1.615 | 0.028-0.030 |
Howlite | 1.586-1.605 | 0.019 |
Zektzerite | 1.582-1.585 | 0.003 |
Amblygonite | 1.578-1.646 | 0.024-0.030 |
Ekanite | 1.572-1.573 | 0.001 |
Anhydrite | 1.570-1.614 | 0.044 |
Augelite | 1.570-1.590 | 0.014-0.020 |
Emerald (beryl) | 1.565-1.602 | 0.006 |
Aquamarine (beryl) | 1.564-1.596 | 0.004-0.005 |
Variscite | 1.563-1.594 | 0.031 |
Beryl (precious) | 1.562-1.602 | 0.004-0.010 |
Tremolite | 1.560-1.643 | 0.017-0.027 |
Vivianite | 1.560-1.640 | 0.050-0.075 |
Serpentine | 1.560-1.571 | 0.008-0.014 |
Larbradorite | 1.559-1.570 | 0.008-0.010 |
Hambergite | 1.553-1.628 | 0.072 |
Pyrophyllite | 1.552-1.600 | 0.048 |
Muscovite | 1.552-1.618 | 0.036-0.043 |
Beryllonite | 1.552-1.561 | 0.009 |
Charoite | 1.550-1.561 | 0.004-0.009 |
Amethyst (quartz) | 1.544-1.553 | 0.009 |
Aventurine (quartz) | 1.544-1.553 | 0.009 |
Rose quartz | 1.544-1.553 | 0.009 |
Citrine (quartz) | 1.544-1.553 | 0.009 |
Prasiolite (quartz) | 1.544-1.553 | 0.009 |
Smoky quartz | 1.544-1.553 | 0.009 |
Rock crystal (quartz) | 1.544-1.553 | 0.009 |
Andesine | 1.543-1.551 | 0.008 |
Cordierite | 1.542-1.578 | 0.008-0.012 |
Oligoclase | 1.542-1.549 | 0.007 |
Talc | 1.540-1.590 | 0.050 |
Scapolite | 1.540-1.579 | 0.006-0.037 |
Petrified Wood | 1.540 | none |
Jasper | 1.540 | none |
Amber | 1.539-1.545 | none |
Ivory | 1.535-1.570 | none |
Apophyllite | 1.535-1.537 | 0.002 |
Tiger’s eye | 1.534-1.540 | none |
Aragonite | 1.530-1.685 | 0.155 |
Agate | 1.530-1.540 | 0.004-0.009 |
Chalcedony | 1.530-1.540 | 0.004-0.009 |
Chrysoprase | 1.530-1.540 | 0.004-0.009 |
Moss agate | 1.530-1.540 | 0.004-0.009 |
Sepiolite | 1.530 | none |
Witherite | 1.529-1.677 | 0.148 |
Milarite | 1.529-1.551 | 0.003 |
Nepheline | 1.526-1.546 | 0.0004 |
Sunstone | 1.525-1.548 | 0.01 |
Amazonite | 1.522-1.530 | 0.008 |
Pearl | 1.520-1.690 | 0.156 |
Ammolite | 1.520-1.680 | 0.155 |
Strontianite | 1.520-1.670 | 0.150 |
Gypsum | 1.520-1.529 | 0.009 |
Orthoclase | 1.518-1.530 | 0.008 |
Sanidine | 1.518-1.530 | 0.008 |
Moonstone | 1.518-1.526 | 0.008 |
Pollucite | 1.517-1.525 | none |
Carletonite | 1.517-1.521 | 0.004 |
Stichtite | 1.516-1.544 | 0.026 |
Thomsonite | 1.515-1.542 | 0.006-0.025 |
Magnesite | 1.509-1.717 | 0.022 |
Scolecite | 1.509-1.525 | 0.007-0.012 |
Leucite | 1.504-1.509 | 0.001 |
Mesolite | 1.504-1.508 | 0.001 |
Dolomite | 1.502-1.698 | 0.185 |
Petalite | 1.502-1.519 | 0.012-0.017 |
Lapis lazuli | 1.500 | none |
Haüyne | 1.496-1.510 | none |
Tugtupite | 1.496-1.502 | 0.006 |
Cancrinite | 1.495-1.528 | 0.024-0.029 |
Celluloid | 1.495-1.520 | none |
Ulexite | 1.491-1.520 | 0.029 |
Yugawaralite | 1.490-1.509 | 0.011-0.014 |
Whewellite | 1.489-1.651 | 0.159-0.163 |
Kurnakovite | 1.488-1.525 | 0.036 |
Inderite | 1.486-1.507 | 0.017-0.020 |
Calcite | 1.486-1.658 | 0.172 |
Coral | 1.486-1.658 | 0.160-0.172 |
Moldavite | 1.48-1.540 | none |
Natrolite | 1.480-1.493 | 0.013 |
Sodalite | 1.480 | none |
Analcime | 1.479-1.489 | none |
Thaumasite | 1.464-1.507 | 0.036 |
Creedite | 1.461-1.485 | 0.024 |
Chrysocolla | 1.460-1.570 | 0.023-0.040 |
Obsidian | 1.450-1.550 | none |
Gaylussite | 1.443-1.523 | 0.080 |
Glass | 1.440-1.900 | none |
Fluorite | 1.434 | none |
Sellaite | 1.378-1.390 | 0.012 |
Opal | 1.370-1.520 | none |
Notes
A double refraction of “none” means the stone is either isometric or amorphous. “Anomalous” means this singly refractive stone may show anomalous double refraction (ADR). Singly refractive gems with high refractive indices may show ADR.