Pleochroism (also: dichroism when limited to two colors) refers to the change in color of light passing through a mineral as the mineral (or light source) is rotated. This effect is brought on by using plane-polarized light, and is evidence of crystallographic orientation. From http://webmineral.com/help/Pleochroism.shtml "The primary cause of dichroism or pleochroism in minerals is due to adsorption of particular wavelengths of light. This selective adsorption of certain wavelengths of light causes the transmitted light to appear colored. This color is a function of the thickness and the particular chemical and crystallographic nature of the mineral. If the adsorption of particular wavelengths of light differs according to the optical path, the phenomena of pleochroism is evident. This is observable in plane-polarized light when the polarizers select the light exiting from the mineral. This color depends on which optical path is viewed. Pleochroic colors are observed and recorded in the mineral data and are generally diagnostic of the particular mineral." So, if a material changes color under rotating polarized light, one can identify the material with a chart. Related information to learn about includes how polarized light behaves, the difference between circular and plane polarized light, anisotropy in minerals, refraction and birefringence.