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.