Pleochroism is the change of color in colored anistropic (double refractive) gemstones. When light enters such a gemstone it will be split in two rays that are perpendicular polarized and each of them travels at a different speed inside the gem and will therefor be refracted in different amounts.
Both rays will experience different forces inside the gemstone which causes the light to be absorped depending on the path they travel. In other words, each ray will be absorped inside the gemstone at different amounts.
The logical consquence of this is that each ray will come out differently colored, depending on the direction. This is named "differential selective absorption".
Sometimes one will encounter notations as "ω = dark green" and "ε = pale green". That refers to the associated colors of each ray (the ordinary ray and the extra-ordinary ray).
Uniaxial gemstones may have two associated colors and this type of pleochroism is termed "dichroism", biaxial gemstone may have three associated colors that type is termed "trichroism".
We quantify pleochroism with a few simple (self-explanatory) phrases:
The tool used to observe this phenomenon is the dichroscope.