The refractive index is a measure of the degree to which light is refracted [bent] as it through a particular stone. Light rays are deflected when they pass from on air [or other medium] into a gemstone, and the measure of this deflection can be used to identify the gem material.

The refractive index is also used in determining how to shape a gemstone. If the RI is known, the gem cutter can adjust the angles at which facets are ground onto the stone to maximize the effect of reflected and refracted light within the stone.