Refractive index, or RI as it is often abbreviated, is one of the most important signatures of a gemstone. Measuring 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 ratio of the velocity of light in air to the velocity of light through a transparent material. If light passes from air into a transparent material at an angle of incidence other than a 90 degree angle, it is deflected at a different angle (the coincident angle) according to the RI. Gemstones with higher RI are generally more brilliant than those with low RI, because more light is returned back out the top or crown of the stone, instead of passing through the bottom or pavilion. Diamond has an RI of about 2.4; quartz, about 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 about 60 varieties of gemstones, sorted in descending order.