You searched for: “pleochroisms
pleochroism, pleochroisms, pleochromatism
1. The property possessed by some crystals of exhibiting different colors, especially three different colors, when viewed along different axes.
2. The phenomenon of different colors appearing when certain crystals are viewed from different directions.
3. 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 traveling at different speeds, they may have the result of differential selective absorption; therefore, when they leave the crystal they have different colors, making the stone appear to be of multiple colors.

Some stones show two colors or shades and are called dichroic, some show three and are trichroic. Gems are sometimes cut and set either to display pleochroism or to hide it, depending on the colors and their potential attractiveness.