What is birefringence in mineralogy ?

Birefringence : definition

Birefringence is the optical property of anisotropic materials to divide an incident light ray into two refracted rays, called "ordinary" and "extraordinary". One of the two refracted rays leaves the anisotropic body with a certain delay with respect to the other. This shift depends on the difference between the refractive indices Ng and Np and on the thickness of the body traversed ; it results in the appearance of colors in optical microscopy (under polarized-analyzed light) which allow the identification of the mineral concerned.
It is easy to verify the existence of birefringence by observing a text through a transparent calcite crystal. The text then appears double, because the two rays each send us an image. As calcite has a high birefringence, the two texts are clearly separated. In gemology, we speak of "doubling". Be careful however, by orienting the crystal perpendicular to its optical axe, there is no more birefringence.