Mineral, UO₂; sp. gr. from 8.5 when massive to 10 for unaltered crystals; hardness 5–6; cubic; it is brownish-black or grey-black; brownish-black streak; sub-metallic, greasy lustre; crystals very rare, the mineral is usually found as botryoidal masses; no cleavage; radioactive; occurs in pegmatites, and in association with monazite, zircon, and tourmaline, in hydrothermal veins together with cassiterite, pyrite, chalcopyrite, and galena, and as a detrital material in alluvials. It is a major ore mineral for uranium metal.