Birefringence is a type of optical property seen in certain crystals and minerals. It is the phenomenon of a material splitting a single beam of light into two different beams, each travelling at different speeds, based on the polarization of the light. When viewed through a polarizing filter, these materials appear to have a color shift, as well as a doubling of the images. This phenomenon is seen in watches with a double-reflective crystal surface, and is used to create a unique effect.
