Uniaxial Stone

Uniaxial Stone
A gemstone possessing one optical axis. Crystals of the hexagonal or tetragonal system are uniaxial.

In uniaxial stones, the optic axis is parallel to the c-axis.
Birefringence (n) In a anisotropic or doubly refractive mineral, birefringence is the difference in values between the highest refractive indices and lowest refractive indices; ...

Uniaxial Gemstones
In uniaxial stones, the optic axis is parallel to the c-axis. Biaxial gemstones have two optical axes that do not line up with the c-axis.

See also: Jewel, Glass, Minerals, Vitreous, Natural