Pleochroism (dichroism, trichroism) is the result of selective absorption of different color components of light as they pass through a crystalline gem material. If a pleochroic gemstone is observed under different light sources, it may display remarkably different colors. Tanzanite is a popular gemstone that shows markedly different colors when viewed under different light sources. The next time you purchase a gemstone note what kind of light is being used to display the stone.