|Refractive Index Chart
Refractive index, or RI as it is often abbreviated, is one of the most important signatures of a gemstone. Measuring the refractive index is one of the first steps in identifying a gemstone, though some gemstones have similar refractive indices and additional tests are required for a definitive identification.
RI is the difference between the speed of light in a vacuum and the speed of light in the gemstone. As light passes through a gemstone, it slows down because a gemstone is more dense than air. This slowing down causes the path of light to change. Refraction is the bending of light as it passes in or out of a gemstone. The angle of refraction in the gemstone determines its RI. Gemstones with a higher RI are more brilliant than those with a low RI. Diamond has an RI of 2.42; quartz, has an RI of 1.54-1.55. The RI of most gemstones is easily measured using a simple optical instrument known as a refractometer.
Some gemstones are singly refractive; they have only one refractive index. Other gemstones - in fact, most - are doubly refractive; they have two different refractive indices. When a beam of light enters a doubly refractive gem, it is split into two beams, each travelling at a different speed and on a different path through the crystal. Birefringence is a measurement of the difference between the two refractive indices in gems that are doubly refractive, and it ranges from a low of .003 to a high of .287. Very few gemstones are singly refractive; in fact, the only well-known gems with that property are diamond, spinel and garnet.
The following chart lists the refractive indices for 143 varieties of gemstones, arranged in descending order.
- First Published: December-23-2007
- Last Updated: June-06-2014
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