Such an effect is called pleochroism, and crystals which show variations in their colour when viewed from different angles, or by transmitted light, are called pleochroic, or pleochromatic -- from two Greek words signifying "to colour more.".    From Wordnik.com.  [The Chemistry, Properties and Tests of Precious Stones] Reference

Iceland-spar is extensively used in the construction of Nicol's prisms for polariscopes, polarizing microscopes and saccharimeters, and of dichroscopes for testing the pleochroism of gem-stones.    From Wordnik.com.  [Encyclopaedia Britannica, 11th Edition, Volume 4, Part 4 "Bulgaria" to "Calgary"] Reference

Biotite, containing haloes, 223; pleochroism of, 235; intensified pleochroism in halo, 235.    From Wordnik.com.  [The Birth-Time of the World and Other Scientific Essays] Reference

Because of Iolite's pleochroism (it appears a different color depending on the angle of the light), they could look through the mineral to determine the sun's exact position and navigate accurately.    From Wordnik.com.  [de.lirio.us] Reference