Staurolite

Stau″ro‐lite (sta̤″rō̍‐līt), n. [Gr. σταυρόσ a cross + -lite.] (Min.) 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.