Kyanite

Kyanite is an aluminum silicate mineral (Al₂SiO₅) that forms during the regional metamorphism of clay-rich sediments. It shares its chemistry with andalusite and sillimanite (polymorphs). It is most often blue and grows as elongated blades whose hardness varies with direction. Synonyms include cyanite and disthene (also spelled distene). It is a raw material for refractory porcelain, and clear, deep blue material is cut as a gemstone.