A mineral having the composition SnO₂. Cassiterite is the principal ore of tin. Its crystals are of the tetragonal rutile structure type, usually in prisms terminated by dipyramids. Twins with a characteristic notch called visor tin are common. Cassiterite is usually massive granular, but may be in radiating fibrous aggregates with reniform shapes (wood tin). The hardness is 6–7 (Mohs scale), and the specific gravity is 6.8–7.1 (unusually high for a nonmetallic mineral). The luster is adamantine to submetallic. Pure tin oxide is white, but cassiterite is usually yellow, brown, or black because of the presence of iron.