Thorianite is a rare thorium oxide mineral, ThO<sub>2</sub>. It was originally described by Ananda Coomaraswamy in 1904 as uraninite, but recognized as a new species by Wyndham R. Dunstan. 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, 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. The largest crystals are usually near 1.5 cm. Larger crystals, up to , have been reported from Madagascar.
Based on color, specific gravity and composition three types of thorianite are distinguished:
Thorianite and uraninite form a complete solid solution series in synthetic and natural material. The division between the two species is at Th:U = 1:1 with uranium possibly making up to 46.50% and thorium ranging up to 87.88%. Rare earths, chiefly cerium, substitute for thorium in amounts up to 8% by mass. Cerium is probably present as Ce<sup>4+</sup>. Complete series is known in synthetic material between CeO<sub>2</sub> - PrO<sub>2</sub> - ThO<sub>2</sub> - UO<sub>2</sub>. Small amounts of Fe<sup>3+</sup> and zirconium also may be isomorphous with thorium. Lead present is probably radiogenic.
Usually found in alluvial deposits, beach sands, heavy mineral placers, and pegmatites.