Pleochroism is the change of color in colored anistropic (double refractive) gemstones. 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".
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).
Uniaxial gemstones may have two associated colors. This type of pleochroism is termed "dichroism". Biaxial gemstones may have three associated colors, and that type of pleochroism is termed "trichroism".
We quantify the quality of pleochroism with a few simple (self-explanatory) phrases:
The tool used to observe this phenomenon is the dichroscope.