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Gemstone's Refractive Index

Gemstone's Refractive Index

Refracive index, the abbreviated is RI, is one of the most important identified of gemstone. Because almost gemstones will have different refractive index, but not all. So if you have to make sure what stone you test, you have to used other method with also. Whatever the refractive index testing have to be the one method you can not deny.

When light passes from one density or type of material (air) into another (the gemstone), it is bent or "refracted." The amount, or degree, that the beam of light is bent will be determined by the density difference between the air and the gem. The measurement used to quantify the amount that a light beam is bent in a given material is known as its "refractive index".

There are two factors when calculating a gem's refractive index: "angle of incident" and "refractive angle." The "incident angle" is the angle of the approaching light as it intersects with the stone's exterior surface. The "refractive angle" is the altered angle of the light as it passes through the stone's interior. The Refractive Index is the ratio of difference between these two angles. Each material has its own unique density and Refractive Index.

The color of a gemstone, and the frequency (color) of the light traveling through it, can also effect its refractive index. As the light's frequency changes, so does its angle of refraction. Higher light frequencies travel through the stone more slowly, while lower frequencies travel faster, causing a spreading effect known as dispersion. As each frequency is reflected it is dispersed throughout the stone's interior at varying speeds and directions. As the scattering and dispersion of light within the stone is increased, so to is the amount of "fire" that is returned to the viewer.

The table below is the list gemstone's RI that arranged from the gemstone that have the lowest refractive index to highest refractive index.

Gemstone	Refractive index	Double Refraction	Birefringence
Opal	1.37 – 1.52	None	N/A
Fluorite	1.434	None	N/A
Obsidian	1.45 – 1.55	None	N/A
Chrysocolla	1.469 – 1.57	Yes	0.023 - 0.040
Moldavite	1.48 – 1.54	None	N/A
Sodalite	1.48	None	N/A
Hackmanite	1.48	None	N/A
Calcite	1.486 – 1.658	Yes	0.172
Coral	1.486 – 1.658	Yes	White and red : 0.160 – 0.172
Lapis lazuli	1.5	None	N/A
Moonstone	1.518 – 1.526	Yes	0.008
Orthoclase	1.518 – 1.53	Yes	0.008
Ammolite	1.52 – 1.68	Yes	0.155
Pearl	1.52 – 1.69	Yes	0.156
Maw sit sit	1.52 – 1.74	Yes	0.015 – 0.02
Amazonite	1.522 – 1.530	Yes	0.008
Lepidolite	1.525 – 1.586	Yes	0.029 – 0.038
Sunstone	1.525 – 1.548	Yes	0.010
Agate	1.53 – 1.54	Yes	0.001 – 0.004
Bloodstone	1.53 – 1.54	Yes	0.001 – 0.004
Carnelian	1.53 – 1.54	Yes	0.001 – 0.004
Quartz ex. Rose, Smoky and Lemon	1.53 – 1.54	Yes	0.009
Fossil coral	1.53 – 1.54	Yes	0.001 – 0.004
Chrysophase	1.53 – 1.54	Yes	0.004 – 0.009
Chalcedony	1.53 – 1.54	Yes	0.001 – 0.004
Tiger’s eye	1.534 – 1.540	None	N/A
Quartz cat’s eye	1.534 – 1.540	None	N/A
Amber	1.539 – 1.545	None	N/A
Scapolite	1.540 – 1.579	Yes	0.006 – 0.037
Peanut wood	1.54	None	N/A
Jasper	1.54	None	N/A
Iolite	1.542 – 1.578	Yes	0.008 – 0.012
Aventurine	1.53 – 1.54	Yes	0.009
Pietersite	1.544 – 1.553	Yes	0.009
Ametrine	1.544 – 1.553	Yes	0.009
Mystic quartz	1.544 – 1.553	Yes	0.009
Onyx	1.544 – 1.553	Yes	0.001 – 0.004
Rutile quartz	1.544 – 1.553	Yes	0.009
Amethyst	1.544 – 1.553	Yes	0.009
Citrine	1.544 – 1.553	Yes	0.009
Charoite	1.550 – 1.561	Yes	0.004 – 0.009
Andesine labradorite	1.551 – 1.560	Yes	0.008
Verdite	1.552 – 1.576	None	N/A
Hambergite	1.553 – 1.628	Yes	0.007
Labradorite	1.559 – 1.570	Yes	0.008 – 0.010
Serpentine	1.560 – 1.710	Yes	0.008 – 0.014
Morganite	1.562 – 1.602	Yes	0.004 – 0.010
Beryl	1.562 – 1.620	Yes	0.004 – 0.010
Goshenite	1.562 – 1.620	Yes	0.004 – 0.010
Veriscite	1.563 – 1.594	Yes	0.031
Aquamarine	1.564 – 1.596	Yes	0.004 – 0.005
Emerald	1.565 – 1.602	Yes	0.006
Seraphinite	1.576 – 1.599	Yes	0.005 – 0.011
Howlite	1.586 – 1.605	Yes	0.019
Larimar	1.595 – 1.645	Yes	0.038
Rhodochrosite	1.6 – 1.82	Yes	0.208 – 0.220
Nephrite jade	1.600 – 1.627	Yes	0.027
Jadeite	1.600 – 1.627	Yes	0.027
Sugilite	1.607 – 1.611	Yes	0.001 – 0.004
Topaz	1.609 – 1.643	Yes	0.008 – 0.016
Gaspeite	1.61 – 1.81	Yes	0.22
Turquoise	1.61 – 1.65	Yes	0.04
Prehnite	1.611 – 1.699	Yes	0.021 – 0.039
Hemimorphite	1.614 – 1.636	Yes	0.022
Tourmaline ex. Chrome, cat’s eye, rubellite, Paraiba and indicolite	1.614 – 1.666	Yes	0.014 – 0.032
Smitsonite	1.621 – 1.849	Yes	0.228
Andalusite	1.627 – 1.649	Yes	0.007 – 0.013
Apatite	1.628 – 1.649	Yes	0.002 – 0.006
Clinohumite	1.629 – 1.674	Yes	0.029 – 0.045
Danburite	1.630 – 1.636	Yes	0.006 – 0.008
Peridot	1.650 – 1.703	Yes	0.036 – 0.038
Malachite	1.655 – 1.909	Yes	0.254
Silimanite	1.655 – 1.684	Yes	0.014 – 0.021
Axinite	1.656 – 1.704	Yes	0.010 – 0.012
Spodumene ex. Kunzite, Hiddenite	1.660 – 1.681	Yes	0.014 – 0.016
Diopside ex. Chrome, star and Tashmarine	1.664 – 1.730	Yes	0.024 – 0.031
Dumotierite quartz	1.678 – 1.689	Yes	0.015 – 0.037
Ruby - Zoisite	1.691 – 1.700	Yes	0.009
Tanzanite	1.691 – 1.700	Yes	0.009
Idocrase	1.700 – 1.732	Yes	0.002 – 0.012
Diaspore or refer to Zultanite	1.702 – 1.750	Yes	0.048
Kyanite	1.710 – 1.734	Yes	0.015 – 0.033
Spinel	1.712 – 1.762
Rhodonite	1.716 – 1.752	Yes	0.010 – 0.014
Azurite	1.720 – 1.848	Yes	0.108 – 0.110
Garnet ex. Pyrote, Star, Rhodolite, Color change and Hessonite	1.720 – 1.756	None	N/A
Grossular garnet ex. Tsavorite, Grossularite and Mali	1.734 – 1.759	None	N/A
Chrysoberyl	1.746 – 1.763	Yes	0.004 – 0.010
Alexandrite	1.746 – 1.763	Yes	0.004 – 0.010
Corundum ex. Sapphire, Ruby	1.762 – 1.778	Yes	0.008
Garnet ex. Almandite and Spessartite	1.77 – 1.82	None	N/A
Zircon	1.810 -2.024	Yes	0.002 – 0.059
Sphene	1.843 – 2.110	Yes	0.100 – 0.192
Melanite	1.88 – 1.94	None	N/A
Andradrite garnet ex. Demantoid and Topazolite	1.88 – 1.94	None	N/A
Cassiterite	1.997 – 2.098	Yes	0.096 – 0.098
Sphalerite	2.368 – 2.371	None	N/A
Diamond	2.417 – 2.419	None	N/A
Hematite	2.94 – 3.22	Yes	0.287