Light may be absorbed differently as it passes through a crystal in different directions. Sometimes the differences are only in degree of absorption or intensity. In other cases, however, different wavelength portions of the transmitted light are absorbed in different directions, resulting in colors. This phenomenon is termed pleochroism. In the case of uniaxial materials, there are only two distinct optical directions and the phenomenon is termed dichroism. Other materials may be trichroic, and the pleochroic colors are sometimes very distinct and strong, and are useful in identification. The pleochroic colors reported for various gems are presented in the order X/Y/Z, separated by slashes.
- Arem, Joel E. Color Encyclopedia of Gemstones, Litton Educational Publishing, Inc.:New York (1977), p. xxv