Thorium dioxide (ThO2), also called thorium (IV) oxide, is a crystalline solid, often white or yellow in color. It was formerly known as thoria. It is produced mainly as a by-product of lanthanide and uranium production. Thorianite is the name of the mineralogical form of thorium dioxide. It is moderately rare and crystallizes in isometric system. The melting point of thorium oxide is 3300 °C – the highest of all known oxides.