Corundum mineral. 19th-century illustration of a form of corundum obtained from Scotland and called 'Scotch Corundum'. Corundum is a crystalline form of aluminium oxide. Its colour varies depending on what impurities are present. Details of the crystal geometry are shown across bottom. This artwork, dated 1804, is from 'British Mineralogy', issued between 1802 and 1817 by British naturalist James Sowerby (1757-1822).