Pitchblende, also called uraninite, is a dense, brown, black, or greenish mineral, the most important source of uranium, radium, and polonium. The composition of pitchblende varies between UO₂ and UO_2.6; thorium, lead, and helium are also present. The blackish and lustrous ore occurs in several varieties: crystallized varieties are called uranianite, massive varieties are called pitchblende. Both varieties are found around the world in veins (often quartz and silver) Principal deposits in Zaire, Bohemia, at Great Bear Lake, Canada, and in the Mountain States. Hardness 5–6; relative density 6.5–8.5 (pitchblende), 8–10 (uranianite).