Trichroa

Trichroa is a rarely used term in mineralogy and gemology referring to a mineral or gem that exhibits three distinct colors depending on the angle of observation due to pleochroism. Pleochroism, in general, describes the phenomenon where a substance appears to be different colors when observed at different angles with polarized light. When three colors are visible, it is specifically referred to as trichroism. Minerals exhibiting trichroism are relatively uncommon. The distinct colors observed are due to the differential absorption of light along different crystallographic axes. This absorption is a function of the mineral's chemical composition and its crystal structure. To accurately observe trichroism, a dichroscope is often used, which allows the simultaneous observation of light from two different directions. In some cases, careful observation with a polarizing microscope may also reveal the effect.