Identifying minerals can be a challenging task, but understanding the unique properties of different minerals can help us narrow down our options. In this article, we’ll explore the concepts of refraction and double refraction and how they can be used in mineral identification.
What Is Refraction?
Refraction is the phenomenon of light being bent as it passes through different materials. When light travels from one medium to another with a different refractive index, it changes direction due to refraction. The degree to which light is bent depends on the refractive index of the material it is passing through.
The refractive index of a material is a measure of how much the material slows down light as it passes through it. Materials with higher refractive indices will bend light more than materials with lower refractive indices. This is why, for example, when light travels from air into water, it is bent at the surface of the water and takes a new direction within the water.
Some common minerals that exhibit refraction include diamonds and garnets. These minerals are transparent and have high refractive indices, which means that they bend light significantly as it passes through them. This is one of the reasons why diamonds and garnets are prized for their sparkling, brilliant appearance.
Measuring Refractive Index
The refractive index of a mineral is an important characteristic that can help us identify and classify different minerals. Refractive index is a measurement of the speed at which light travels through a particular mineral. This measurement of light can then be used to help in mineral and gemstone identification.
There are various methods for measuring the refractive index of a mineral, but most of them involve using specialized optical instruments and ultimately applying a certain amount of math. fortunately, advances have been made and it’s become much simpler to measure mineral refraction at home with the use of a common instrument called a refractometer.
How a Refractometer Works
To measure the refractive index of a mineral using these instruments, a beam of light is passed through the mineral and the degree to which it is bent is measured. The refractive index is then calculated based on the amount of bending that occurs.
Measuring the refractive index of a mineral can be a complex task that requires specialized training and equipment. However, the work has already been done by competent scientists, and tables of refractive indices for various minerals have been compiled and are widely available. These tables provide values for the refractive indices of different minerals, making it easier to identify minerals based on their refractive properties. I’ve provided a list of common minerals and their refractive indices at the end of this article.
Read More: How To Use a Refractometer to Identify Rocks, Minerals and Gemstones
What Is Double Refraction?
Double refraction, also known as birefringence, is a property that is exhibited by certain minerals when a beam of light is split into two paths as it passes through the mineral. This occurs because the mineral has a different refractive index in different directions, which causes light to be refracted differently depending on its direction of travel.
Minerals that exhibit strong double refraction include peridot, zircon, sphene, epidote, tourmaline, kunzite, ruby, sapphire, topaz, amethyst, quartz, emerald, aquamarine, and chrysoberyl. These minerals can be distinguished from other minerals based on their strong double refraction.
Double refraction can be observed by viewing the edges of the rear facets of a mineral through a lens. When double refraction is present, the edges of the facets will appear double-lined. This simple test can be used to distinguish minerals that exhibit strong double refraction, such as zircon, from minerals that do not, such as diamond.
Using Refraction and Double Refraction in Mineral Identification
Refraction and double refraction are just two of the many tools that can be used to identify minerals. It’s important to use multiple methods and consider all available information when trying to identify a mineral, as no single method is foolproof.
Here are a few tips for using refraction and double refraction in mineral identification:
- Compare the mineral to known specimens: If you have access to a collection of known minerals, you can compare the refractive properties of the mineral you are trying to identify to those of known minerals. This can help you narrow down your options and make an educated guess about the identity of the mineral.
- Use a lens to observe double refraction: As mentioned earlier, minerals that exhibit strong double refraction can be distinguished from other minerals by observing the edges of the rear facets through a lens. If the edges appear double-lined, the mineral is likely one that exhibits strong double refraction.
- Consult tables of refractive indices (below): As mentioned earlier, tables of refractive indices for various minerals are widely available and can be a helpful resource for identifying minerals based on their refractive properties. By comparing the refractive index of the mineral you are trying to identify to the values listed in the tables, you can get a better idea of what the mineral might be.
By using these tips and considering all available information, you can increase your chances of accurately identifying minerals using refraction and double refraction.
List of Mineral Refractive Indices and Double Refraction Values
Credit for the following information: International Gem Society
| Mineral | 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 |
| 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 |
| Cordierite | 1.522-1.578 | 0.008-0.018 |
| Iolite (cordierite) | 1.522-1.578 | 0.008-0.018 |
| 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 |
| Villiaumite | 1.327 | anomalous |
Perfect for every rock collector and geology enthusiast to have on hand. This valuable reference covers more rocks and minerals in North America than any other available guide.
