Staurolite
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Staurolite is a reddish 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 the chemical formula: ... Wikipedia
Hardness: 7 – 7.5
Birefringence: δ = 0.009 – 0.015
Cleavage: Distinct on
Color: Dark reddish brown to blackish brown, yellowish brown, rarely blue; pale golden yellow in thin section
Crystal class: Prismatic (2/m); (same H-M symbol)
Crystal habit: Commonly in prismatic crystals
Fracture: Subconchoidal