Since light waves are absorbed at different rates along different crystal axes of a doubly refractive gemstone, different axes can appear to be different colors; this is called pleochroism. Pleochroic gemstones may have a different color when looked at from different directions.

This can be simply observed in a rectangular shaped dark green tourmaline: if you look down one end one color, usually green, is visible and another color, usually brown, is visible when you look at it sideways. Iolite is another gemstone where pleochroism is easily observed: it appears blue when looking at it through the table, but may look completely colorless when viewed from the side! In most pleochroic gemstones this is not so obvious.

A gemological tool, the dichroscope, aids in observing pleochroism. Stones that have two pleochroic colors are dichroic. Examples of dichroic gems are tourmaline, amethyst, rubies and sapphires. When a gem exhibits three pleochroic colors it is called trichroic. Trichroic gems include tanzanite, andalusite and iolite.

Pleochroism creates unique color qualities in gemstones; it can add the richness of a gemstone. It is necessary to examine pleochroic gemstones at all angles to see the full color potential. This is most obvious when looking at tourmalines and andalusite where the pleochroic colors are easily observed. Tourmalines that appear to have a pale orange color generally have a pink color obvious in one axis and a yellow color apparent on the other axis; if the cutter had oriented the stone in another direction, the color would appear differently.