Pleocroism

Pleocroism is the property of a crystal to absorb different frequencies of light from different angles. Pleocroitic crystals can appear different colors along different axes.

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Pleocroism appears to be strongly tied to the structure of the crystal. For example hexagonal crystals may show two colors but never more. Triclinic crystals can show as many as three colors.

Detection

Pleocroism is also used as one factor among many in identifying gemstones. However, many gems have a sufficiently small disparity in the light absorption along different angles that one cannot detect this property with a naked eye. Often a polariscope is used and the crystal or gem is rotated over the polarizer in order to detect the change of color. For example in this way a ruby can be distinguished from a red spinel which is not pleocroitic.

Some gems are sufficiently pleocroitic to allow the effects to be seen with the naked eye, however. For example Iolite appears bluish purple from one angle and nearly colorless from another. Green tourmaline often appears black if viewed along its C axis. Some other tourmalines will appear very different colors if viewed in different angles.