Pleochroism is the change of color in colored anistropic (double refractive) gemstones when viewed from different directions or when viewed though plane polarized light. When light enters such a gemstone, it will be split into two rays that are perpendicular polarized. Each of the two rays travels at a different speed inside the gem, so they will be refracted differently.
Both rays will experience different forces inside the gemstone which cause the light to be absorbed depending on the path they travel. In other words, each ray will be absorbed inside the gemstone in different amounts.
The logical consequence of this is that each ray will be differently colored, depending on its direction. This result is named "differential selective absorption".
Uniaxial gemstones may have two associated colors. This type of pleochroism is termed "dichroism". Dichroic minerals always form in the hexagonal, trigonal, or tetragonal crystal systems.
Sometimes one encounters notations such as "ω = dark green" and "ε = pale green". These notations refer to the associated colors of each ray (the ordinary ray and the extra-ordinary ray respectively).
Biaxial gemstones may have three associated colors, and that type of pleochroism is termed "trichroism". Trichroic minerals always form in the orthorhombic, monoclinic, or triclinic crystal systems.
Although an incident ray of light is also split in two rays in biaxial gemstones, they may vibrate in three different directions. As a result, light will be absorbed differently in 3 directions.
For biaxial minerals the notations are nα, nβ and nγ (relating to the X, Y and Z vibrational directions).
We quantify the quality of pleochroism with a few simple (self-explanatory) phrases:
The gemological tool used to observe this phenomenon is the dichroscope. But one can also use the light emitted by a computer monitor to view pleochroism as a computer screen emits plane polarized light! If one rotates their gem in front of a white screen, like this one, pleochroism can be determined.