A mineral, thorium silicate, in which the element thorium was discovered in 1828. Thorite is tetragonal in crystallisation and has a crystal structure identical with that of the nesosilicate zircon, ZrSiO₄. The idealised chemical formula of thorite is ThSiO₄. All natural material departs widely from this composition owing to the partial substitution of uranium, rare earths, calcium, and iron for thorium. Structurally, thorite usually has completely lost its crystallinity because of radiation damage from the contained uranium and thorium (metamict state). The specific gravity ranges between about 4.3 and 5.4. The hardness on Mohs scale is about 4½. The colour commonly is brownish yellow to brownish black and black. Thorogummite is a chemical variant of thorite with the same crystal structure and very similar properties. It is deficient in silica and contains small amounts of OH in substitution for oxygen.