Birefringence is an optical property of crystals, in where the stone has more than one index of refraction. Note that this does not necessarily mean that the stone is transparent or translucent, as indices of refraction do not have to be restricted to visible light, and can function just as well with x-rays on an opaque crystal. Many trigonal and orthorhombic crystal structures exhibit this behavior, as well as hexagonal systems.

Note, this is not restricted to 2 indices, as can be seen with Beryl, which has 4, but any more than 2 is exceedingly uncommon.