The splitting of light into two light rays may result in an effect called pleochroism in some colored transparent uniaxial and biaxial minerals. Pleochroism is produced because the two rays of light are differencially absorbed as they pass through the colored anisotropic mineral. Because the absorption is different for the two light rays, the wavelengths and colors of the two light rays are different when they emerge from the crystal. The color of ruby and sapphire (uniaxial corundum) is caused in part by a blending of the two colors. Biaxial minerals, or those that crystallize in the orthorhombic, monoclinic, and triclinic crystal systems, can have three distinct colors. Tiny crystals of biaxial ivaite, the black mineral in the photograph to the left, is strongly pleochroic (yellow, dark brown, and dark green).
Pleochroism12 08 2010