In Bancroft, Ontario Biotite forms large crystals with green apatite and hornblende. Color is black to brown and yellow with weathering. Luster is vitreous to pearly. Transparency crystals are transparent to translucent. Also as lamellar or granular rock forming masses providing the luster for most schists and gneiss.
Cleavage is perfect in one direction producing thin sheets or flakes.
Fracture is not readily observed due to cleavage but is uneven. Hardness is 2.5. Streak is white. Associated Minerals are quartz , feldspars , apatite , calcite , hornblende , garnets and schorl . Best Field Indicators are crystal habit, color, cleavage, elastic sheets and associations.