The Borneo diamonds are likewise distinguished by their exceptional hardness. They mostly occur by the river Landak, near Pontianak on the west coast of the island. They are found in a layer of rather coarse gravel, variable, but rarely exceeding a yard (1 m.), in depth, and are associated with corundum and rutile, together with the precious metals gold and platinum. Indeed, it is no uncommon sight to see natives wearing waistcoats ornamented with gold buttons, in each of which a diamond is set. The diamonds are well crystallized and generally of pure water; yellowish and canary-yellow stones are also common, but rose-red, bluish, smoky, and black stones are rare. They seldom exceed a carat in weight; but stones of 10 carats in weight are found, and occasionally they attain to 20 carats. In 1850 a diamond weighing 77 carats was discovered. The Rajah of Mattan is said to possess one of the purest water weighing as much as 367 carats, but no one qualified to pronounce an opinion regarding its genuineness has ever seen it. In Rhodesia small diamonds have been found in gravel beds resting on decomposed granite near the Somabula forest, about 12 miles (19 km.) west of Gwelo, in association with chrysoberyl in abundance, blue topaz, kyanite, ruby, sapphire, tourmaline, and garnet. The occurrence of diamond in German South-West Africa is very peculiar. Large numbers of small stones are found close to the shore near Luderitz Bay in a gravelly surface layer, which is nowhere more than a foot in depth. They are picked by hand by natives and washed in sieves. In shape they are generally six-faced octahedra or twinned octahedra, simple octahedra being rare, and in size they run about four or five to the carat, the largest stone as yet found being only 2 carats in weight. Their colour is usually yellowish. Several isolated finds of diamonds have been reported in California and other parts of the United States, but none have proved of any importance. The largest stone found weighed 23¾ carats uncut; it was discovered at Manchester in Virginia. CHAPTER XVIII HISTORICAL DIAMONDS