Pleochroism is an optical phenomenon where due to double refraction of light by a colored gem or crystal, the light is divided into two paths which are polarized at a 90° angle to each other. As the divided light follows different paths within the stone and are traveling at different speeds, they may have the result of differential selective absorption, thus when they leave the crystal they have different colors, making the stone seem to be of different colors.
Some stones show two colors or shades and are called dichroic, some show three and are trichroic. Gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their attractiveness.
Pleochroism is an extremely useful tool in mineralogy for mineral identification, since minerals that are otherwise very similar often have very different pleochroic color schemes. In such cases, a thin section of the mineral is used and examined under transmitted light in a microscope.