Cordierite

The gemstone variety is called iolite or dichroite. Cordierite, or iolite, is a magnesium and iron aluminosilicate mineral of widespread occurrence. It forms short, blue prismatic crystals, embedded grains, and compact masses, and has poor cleavage in one direction in addition to a vitreous luster. Although found in some igneous rocks, cordierite most typically occurs in metamorphic rocks, such as gneiss and schist. The gemstone variety, dichroite, shows strong pleochroism -- it is colorless when viewed from one direction and violet when viewed from the other.