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 travelling 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 called pleochroic (or trichroic). Gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their attractiveness.
Dichroic gems generally always have hexagonal, trigonal, or tetragonal crystallography. Pleochroic gems generally always have orthorhombic, monoclinic, or triclinic crystallography. Isotropic gems are never pleochroic.
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.
Some gems displaying pleochroism:
Actinolite: yellow, dark green
Alexandrite: (strong) deep red, orange yellow, green
Andalusite: (strong) olive green, brick red; also light red, yellow, pale yellow
Chondrodite: yellowish brown, reddish brown, dark red, brownish yellow
Chrysoberyl: (strong) yellow, brown (also see Alexandrite above)
Diaspore: (strong) pale green, rose red; also colorless, violet-blue, dark red
Enstatite: (strong) pink to green
Hiddenite: (strong) green, blue-green, pale green, colorless
Iolite: (strong) colorless, violet or blue; also gray, pale yellow
Kornerupine: (strong) green, yellowish brown; also brown, greenish yellow
Kunzite: (strong) pink, purple-violet, colorless
Kyanite: (strong) violet-blue, colorless, cobalt-blue; also blueish green and green
Sillimanite: (strong) pale yellow, green; also pale to dark brown, blue, violet-blue
Sinhalite: (strong) pale brown, greenish brown, dark brown
Sphene: (moderate to strong) pale yellow, greenish yellow, orange-brown
Staurolite: (strong) colorless, yellow, red, golden yellow
Tanzanite: (strong) purple, blue; also yellow or green (Zoisite)
Zoisite: (strong) yellow, green