Pleiochromism.

Pleiochromism. In some crystals, such as some specimens of topaz, three distinct colours may be observed on looking through them along three rectangular axes. In intermediate directions intermediate tints may be observed; and this property is that of pleiochromism or polychromism. In some other cases a similar range of intermediate tints may be observed, lying between two extreme tints visible along two axes (dichroic crystals—e.g. iolite or sapphire d'eau, and some specimens of mica).