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.