| A biaxial crystal, one having two optic axes, will exhibit trichroism, in which three colours, sometimes called face colours, may be observed. |
| In the more familiar blue variety, stones exhibit blue, violet and reddish orange but in fancy colors the trichroism is as follows. |
| This is one of the features of the stone and is due to Tanzanite's trichroism. |
| The cutter must orient the rough carefully, taking iolite's trichroism of blue, gray and near colorless into account. |
| Most tanzanite is treated with heat to deepen the primary colors, and this subdues the trichroism slightly. |
