Strange as it may seem, the diamonds stick fast to the grease; the other material is washed away. It has been found by trial that grease will cling to the precious stones but to nothing else. After a few hours the grease with the diamonds is scraped off the tables and steamed in perforated vessels to separate them. [Illustration: Sorting gravel for diamonds in the Kimberley mine] One of the De Beers mines has been worked to a depth of about two thousand feet with no diminution in the quantity or quality of the diamonds. The "pipe" or plug of blue-stuff shows no signs of giving out. Nature, in her underground laboratory, works in a mysterious way, baffling the astutest students of science to find the process by which she is able to manufacture such beautiful gems as the diamond. Many theories have been propounded to explain the genesis of the diamond, the most plausible one being that the crystallization of the carbon is due to a very high temperature and tremendous pressure acting on the carbon in a liquid form deep down beneath the earth's surface. The crystals, intermingled with much foreign matter, are afterward projected upward, filling these great volcanic pipes. In order to produce the most beautiful effect, diamonds are usually cut into one or another of three different forms, namely, rose, table, and brilliant, the shape and size of the stone determining which form is best. The double-cut brilliant is the most common form at the present day. The general form of rough, crystallized diamonds is that of two square pyramids joined at their bases. The crystals are oftenest found octahedral and dodecahedral--that is, eight and twelve sided, and the diamond-cutter takes advantage of these forms in shaping the diamond. The modern lapidary must have a perfect knowledge of optics and be a skilful stone-cutter. The numerous planes or faces which he cuts on the surface of the diamond are called facets. In the treatment three distinct processes are utilized--cleaving, cutting, and polishing. The lapidary must study the individual character of each stone and determine whether to cleave or grind off the superfluous matter so as to correct flaws and imperfections. All this calls for the judgment which comes only with long experience, for if the cutter errs he may ruin a priceless gem.