Depending on the nature of the mineral, when a ray of light enters a stone, it might be refracted (deviated) and travel in a single direction, or it might be doubly refracted, that is, split into two rays of light, deviated differently, which travel in two different directions within the stone. In the first case, the stone would be singly refractive (SR) or isotropic, while the latter would be doubly refractive (DR) or anisotropic. These properties greatly affect how the mineral interacts with light and the appearance of colours.
The polariscope is an instrument that helps to identify this optical character. It consists of a basic combination of a light source and two rotating polarising filters. The stone is placed in between the filters with the light shining through the filters and the stone from underneath. The stone and the upper filter are then rotated horizontally in various positions to assess the optical nature of the stone. This test, though not always conclusive, is another vector that assists in the gem identification process.
