Pleochroism in Iolite (cordierite)
Iolite, the gemstone variety of the mineral cordierite, is pleochroic. That means it changes color depending upon the direction of observation.
As this video plays, note how the color of the material changes with each 90 degrees of rotation, from violet blue to clear and back to violet blue.
When iolite is cut into a faceted stone, the cutter must study the stone so that it will be cut with the table perpendicular to the observation direction of its best color.
Learn more at... http://geology.com/minerals/cordierit...