staurolite

A brownish to black mineral, chiefly (FeMg)₂Al₉Si₄O₂₃(OH), often having crossed intergrown crystals and sometimes used as a gem.

A silicate of aluminium and iron occurring in reddish- to yellowish-brown or brownish-black prismatic crystals. These crystals are often twins, in the form of a cross, whence it is called cross-stone. Also staurotide, qrenatite.

A mineral of a brown to black color occurring in prismatic crystals, often twinned so as to form groups resembling a cross. It is a silicate of alumina and iron, and is generally found imbedded in mica schist. Called also granatite, and grenatite.

A dark brown nesosilicate mineral that has crystals that cross and intergrow