Thorianite is a rare mineral, discovered by W. D. Holland, and found in the gem-gravels of Sri Lanka, where it occurs mostly as small, heavy, black, cubic crystals. Most deposits of Thorianite Crystals are Alluvial, so you see mostly water worn crystals. It was so named by W. R. Dunstan, on account of its high percentage of thorium (about 70% ThO2); it also contains the oxides of uranium, lanthanum, cerium and didymium. Helium is present, and the mineral is slightly less radioactive than pitchblende but harder to shield due its high energy gamma rays.
The largest crystals (sizes usually up to around 1,5 cm; very rare are sizes greater than 2,5 cm; largest is 6 cm and 2,2 kilos) came from Madagascar. Today there is no mining for Thorianite there.