pleochroism

Pleochroism occurs when a crystal absorbs light differently along different orientations within the crystal. Meaning, when looking down one axis of the crystal, light enters randomly polarized (there is no preference for the direction that the amplitudes of the waves are travelling). As the light passes through the crystal, only light which is in a particular orientation is absorbed, allowing other light to pass through. When this occurs for a particular band of visible light, you get crystals that are differently colored when viewed along different directions. This is all very confusing. The best explanation I could find on the web after a brief search is still a bit confusing, but much more elegantly worded with a touch of bad grammar. They're Spanish, so I will give them a free pass. I cut some tiny pieces of the crystal with a razor and tried to line them up so you can see the two different colors. These two crystal fragments (right) were cut from the same single crystal (largest crystal from the left picture). These are technically dichroic because they display only two colors, although pleochroism would only be used for crystals which displayed three different colors, but this is never called "trichroic" for some reason.