The property shown by a mineral possessing two refractive indices, so that a double image is produced through it. Interference colours are caused when viewed in polarized light.

When viewed through a microscope, a thin section (around 50 microns thick) of rock will show various minerals. The colour of these minerals can change depending on the crystal structure: quartz can change from grey to white, olivine from grey, to red, to blue when rotated.

Roughly, this is caused by the light being filtered through the mineral at different angles, due to the crystalline structure of the mineral. A very useful diagnostic tool, since most minerals have different refractive indices.

The most striking and easily reproduced example of birefringence would be calcite. Holding a crystal of calcite up to a sheet of paper with text on it will result in a double image of the words, demonstrating how the light is refracted in two directions.