The composition of biotite is K(Mg,Fe)3[AlSi3 O10](OH,F)2. It crystallises in the monoclinic crystal system, and the crystals are blocky or conical columnar in shape. It often has a pseudohexagonal platy form. Its colour varies from black or dark brown to reddish brown and is rarely green or white, always with white streaks. Its diaphaneity ranges from transparent to opaque, and it has a pearly or vitreous lustre. The specific gravity is 2.7–3.4, the Mohs hardness is 2.5–3, and the fractures are uneven. Biotite is usually found in igneous and metamorphic rocks.