Gem: The Definitive Visual Guide
“One of the most desirable qualities in a gemstone is beauty, and an important part of this is the stone’s color. In gems, color is caused when light is absorbed within the crystal, or refracted--changing direction as it passes through the gem. White light is composed of many colors; when one or more of these colors is absorbed, the remaining light emerging from the gem is colored. This can be brought about by the presence of trace elements that cause certain wavelengths to be absorbed, or can be a part of the gem’s chemical structure,”
-Gem: The Definitive Visual Guide by The Smithsonian