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[Roger Warin]

Black Rock Mine – Hotazel, RSA.
A small specimen of calcite showing the double refraction inducing a splitting of faces.
Roger.

[Roger Warin]

Double Refraction and Reflection.
In a crystal of calcite, two things happen. The light ray can be reflected on a fault plane and it is also doubly refracted (through the air in the mineral) in two rays, one is called the ordinary ray, the other is the extraordinary ray.
Here is a cleavage rhombohedron illuminated by a green laser (532 nm). The entire crystal takes the green color, but also there occurs a fluorescence which materializes the various rays diffracted or reflected.
Roger.