Page:Encyclopædia Britannica, Ninth Edition, v. 7.djvu/183

DIAMOND 165 diamond-digging has become a regular branch of industry to a large population ; and it is probable, though no very accurate estimate can be formed, that nearly fifteen million pounds sterling worth of diamonds have been obtained from this district since their discovery. The largest diamond from the Cape we have seen mentioned is the Stewart, of 2885 carats, found on the Yaal river in 1S72. It was an irregular octahedron of the purest water, and l inch in diameter, and is of a light yellow since cut. There has been much speculation regarding the mode of origin of these gems, but hitherto leading to no certain result. Newton conjectured that the diamond was " an unctuous substance coagulated ; " Jameson thought it might be a secretion from some ancient tree, like amber ; and Brewster also traced it to a vegetable source. Lavoisier, Guyton-Morveau, and others observed black specks when diamonds were burned, which were considered as uncrys- tallized carbon. Petzhold, in 1842, also supported this view, affirming that he had found vegetable .cells in the ashes of diamonds. Goeppert, in his Haarlem Prize Essay, in 1863, supported the same view, both from supposed plant tissues and from other inclosures in diamonds, but admitted that the evidence was not free from doubts. Liebig and others have explained its origin by a slow pro cess of decomposition in a fluid rich in carbon and hydro gen. On the other hand, the occurrence of the diamond in the itacolumite or mica slate, and more recently in or near igneous rocks, as at the Cape, has tended to favour the view that it owes its origin to heat or metamorphic action, as is the case with graphite. But this, as graplrte also shows, does not preclude ths idea that originally it may have been, like amber, some peculiar vegetable product, subsequently altered and crystallized. It may here also be mentioned that all attempts to produce diamonds artificially have hitherto failed. Diamonds are chiefly used and valued as ornamental stones, and for this purpose they are cut in various forms according to the original shape of the crystals. It is probable that the Indians knew some method of doing this at an early period, and it is said there were diamond- polishers in Nuremberg even in 1373. Berghem of Bruges has the credit of having first used, in 14-56, their own powder for this purpose. He found that by rubbing two diamonds on each other their surfaces were polished and facets formed, and acting on this hint, he employed diamond powder and a polishing wheel. His countrymen continued to follow out the art with great success, but some two centuries ago the English cutters were the more celebrated. The trade then reverted to Holland, but is again returning to Britain, where many of the finest stones are cut. The method has undergone little change, and is still chiefly effected by the hand, partly by rubbing one stone on another, partly by a wheel and diamond powder. Where there are flaws or large pieces of value to be removed, they are occasionally cut by iron wires armed with the powder, or split by a blow of a hammer and chisel in the direction of tho natural cleavage. The latter is, however, a danger ous process, as the diamond is very brittle, and many valu able gems have been thus destroyed. When reduced to a proper form; the facets are polished on a lapidary s wheel. The process demands not only great skill but much time and labour. The period required to reduce a stone of 24 or 30 carats to a regular form extended formerly to at least seven or eight months of constant work, and in the case of the Pitt diamond two years were needed ; but the time is now greatly shortened by the use of machinery driven by steam. Jewellers have long cut diamonds in three forms the brilliant, the rose, and tables. The brilliant is most esteemed, as giving highest effect to the lustre, and implying less reduction of the stone. It is, as it were, a modification of the primary octahedron, the most common form of crystal, and is shown in its first form in figs. 6 and 7, and with the full number Fig. 6. Fig. 7. Fig. 9. FIGS. 6-9. Showing cutting of brilliants. of facets in figs. 8 and 9. Figs. 4 and 6 show the upper surfaces, with the table, or principal face, in the middle, surrounded by the bezil, or upper faces, lying between its edge and the girdle, or common base of the two pyramids. The lower facet corresponding to the table is named the collet, and the whole portion below the girdle the collet side. The portion removed to form the table (generally T 5 ^-ths) and the collet (A-th) is shown iu fig. 10 Brilliants are usually set open, both the upper or table side and the lower collet-side being exposed. The rose cut (upper view, fig. 11 ; lateral view, fig. 12) is given to stones which have too little depth to be cut as brilliants ; it has the whole upper curved surface covered with equilateral tri- Fig. 10. angles. The table diamond, figs. 13 and 14, the least beautiful, is adopted for broad stones oi trifling depth, Fig. 12. FIGS. 11 aiid 12. Rose cut. showing a series of four-sided facets above and below the girdle. Recently brilliants are cut in the star form Fig. 14. Fig. 13. FiGS. 13 and 14. Table diamond. (taille a e"toile), with the table above only one-fourth

the diameter, and thus with less loss of weight. There