[7] It was so named by Dunstan on account of its high percentage of thorium; it also contains the oxides of uranium, lanthanum, cerium, praseodymium and neodymium.

Helium is present, and the mineral is slightly less radioactive than pitchblende,[8] but is harder to shield due to its high energy gamma rays.

It is common in the alluvial gem-gravels of Sri Lanka, where it occurs mostly as water worn, small, heavy, black, cubic crystals.

Based on color, specific gravity and composition three types of thorianite are distinguished:[9] Thorianite and uraninite form a complete solid solution series in synthetic and natural material.

Usually found in alluvial deposits, beach sands, heavy mineral placers, and pegmatites.