a bluish or violet, crystalline mineral, a silicate of aluminum, iron and magnesium; it is used as a gem, magnesium aluminium silicate with some iron, typically occuring in thermally matamorphosed rocks and in some gneisses. Often shows cyclic twinning. Iolite is a name often used for gem varieties and the mineral is also sometimes called dichroite from its strong blue to colourless dichroism.