When a ray of light passes from one medium to another, or from the air
to a crystal it is bent or refracted; this is called single refraction
and takes place in the diamond, spinel, and garnet.
Most of the other transparent precious stones possess double
refraction—that is, the ray of light enters the crystal and divides
into two parts, one following the ordinary laws of refraction, while
the other part or extraordinary ray does not obey the usual law.
There are precise methods for measuring the indices of refraction, but
they are not applicable to polished gem stones.