What is refringence in mineralogy ?

Refringence : definition

Refringence is the property for a body to refract a light ray, ie to make it change direction. The importance of this deviation depends on the value "n" of the refractive index of the body crossed.

In mineralogy, only the cubic and amorphous minerals are refractive, all the others are birefringent, ie they divide in two the light ray which crosses them, giving rise to an ordinary ray and an extraordinary ray.