Staurolite: Gemstones Information
Staurolite is (Fe2,Mg)2Al9(Si,Al)4O20(O,OH)4 and forms highly characteristic monoclinic, pseudo-orthorhombic prismatic, typically rough reddish brown crystals, often forming 60° twins. A few crystals are just about transparent but the majority are translucent with hardness 7–7.5 and SG 3.68–3.83. The RI for the alpha, beta and gamma rays is 1.736–1.747, 1.742–1.753, 1.748–1.761, biaxial positive with birefringence 0.011–0.015. Distinct pleochroism gives colourless to yellow and reddish brown. Absorption bands may be seen at 632 and 610 nm (these in zinc rich material) and sometimes at 531.5 nm.Staurolite occurs most commonly in schists and gneisses; cross-like twins from Pizzo Forno and Alpe Piona, Ticino, Switzerland, and large crystals from the Finistere and Morbihan areas of France. Some rare facetable crystals from Brazil, perhaps from Rubellita and Ardenella, Minas Gerais. Reddish brown to brownish black staurolite is reported from Myanmar.