Double Refraction in Minerals

Double refraction is the splitting of a single ray of light into two refracted rays when passing through an anisotropic mineral. All anisotropic minerals exhibit double refraction.
Double refraction can be practically observed by placing a transparent Calcite (Iceland spar) crystal on a sheet of paper with a black dot on it. When the dot is viewed through the crystal, two dots are seen. Also when the crystal is either rotated or tilted one of those images moves while the other remains stationary. This implies that the mineral has two refractive indices, one constant and one that varies with direction. The fixed image is called the "Ordinary image" and the moving image is called the "Extraordinary image". Similarly the rays that produce the images are also named "O-ray" and "E-ray". The Refractive indices are also identified as RIO and RIE. The relative magnitudes of RIO and RIE determine the Optic Sign of the mineral.

calcite crystal showing double refraction. image from : wikipedia