Andalusite

 Andalusite

 Andalusite is an aluminium nesosilicate mineral with the chemical formula Al₂SiO₅. Andalusite is trimorphic with kyanite and sillimanite, being the lower pressure mid temperature polymorph. At higher temp and pressure, andalusite may convert to sillimanite. Thus, as with its other polymorphs, andalusite ia an aluminosilicate index mineral, providing clues to depth and pressures involved in producing the host rock.Andalusite is a strongly pleiochroic gem, which means that is has different colors when viewed from different directions.