This is called dichroism or pleochroism . Cordierite, the most famous dichroic mineral is bluish-purple but turns gray when rotated or viewed at a different angle . Mineral colors may be artificially enhanced in various ways. A few minerals, especially agates , are periodically dyed to enhance their color. How to use color as an identification mark Color alone will not normally identify a mineral, but it can be a very helpful factor.
Many minerals come in a great diversity of colors, and many different minerals have similar colors.
Such minerals are known as monochromatic minerals. Together with color, a crystal shape can identify most of the better known minerals. It is best to observe the mineral in strong light, preferably daylight. Make sure not to observe a tarnished or discolored surface. One more thing to bear in mind is that some minerals can have a questionable color.