Staurolite is a red brown to black, mostly opaque, nesosilicate mineral with a white streak. It crystallizes in the monoclinic crystal system, has a Mohs hardness of 7 to 7.5 and a rather complex chemical formula: Fe²⁺₂Al₉O₆(SiO₄)₄(O,OH)₂. Magnesium, zinc and manganese substitute in the iron site and trivalent iron can substitute for aluminium.