An important metamorphic index mineral and one of the three Al₂SiO₅ mineral polymorphs, the other two being sillimanite and andalusite; sp. gr. 3.5–3.6; hardness 4–7; triclinic; light to darker blue, sometimes green or yellow, occasionally black; vitreous lustre; crystals usually long, columnar, often flattened and bladed, with occasional stellate intergrowths; cleavage pinacoidal, good {100}, {010}, parting {001}; occurs in regional metamorphic rocks that have been subjected to high pressures and low to moderate temperatures, and may be found in schists or gneisses associated with staurolite and garnet.