In Germany and France and in the United States, the fabrication of rock-crystal is accomplished almost entirely by machinery. The crystal to be shaped into a ball is placed against a semicircular groove worn in huge grindstones. This is illustrated in the case of the method practised in Oberstein, Germany. The workman has his feet firmly braced against a support, and, resting upon his chest, presses the crystal against the revolving grindstone. It is unnecessary to add that the practice is extremely unwholesome and develops early consumption among the workers. A constant stream of water is kept flowing over the stone so that the crystal shall always be moist, as the friction would otherwise hurt it, and the subsequent addition of water would be liable to cause a fracture. The final polishing is done on a wooden wheel with tripoli, or by means of a leather buffer with tripoli or rouge.[323] There are three fine crystal balls in the collection of the American Museum of Natural History. One, apparently perfect, measures 5½ inches in diameter and was cut from a crystal found in Mokolumne, Calaveras Co., California; the second is 6½ inches in diameter and is from the same locality, but not entirely perfect. These were shown in the department of the Tiffany Collection prepared by the author, and were exhibited at the Paris Exposition of 1900 as part of the J. Pierpont Morgan gift to the American Museum of Natural History. Another fine crystal ball is now to be seen in the American Museum of Natural History, New York; this was donated to the institution. It measures 4-11/16 inches in diameter, is of wonderful purity, and the cutting has been executed with such a high degree of precision that an ideally perfect sphere has been produced.[324]