Pleochroism of minerals is an optical phenomenon that affects certain minerals. It is characterised by a variation in the colour of the mineral depending on its orientation. The different axes are oriented at 90° to each other.
Pleochroism can be expressed in a more or less visible way. We speak of strong, medium and weak pleochroism.
When the mineral shows a variation between two colours, it is called dichroism. In the case of a variation between three colours, it is called trichroism.
Example of the pleochroism from brown to violet of an axinite from the Oisans.
Example in image of the pleochroism from green to brown of a verdelite (green tourmaline variety elbaite).
Here are some minerals that show pleochroism: