After the stone is engraved the polish is restored to the fiat surface by a pewter polishing disk or lap fed with rotten stone and water. The engraved portions are polished with great care, first by using in the mill copper tools charged with diamond powder; this buries itself more deeply in the copper than in the iron tools, and a smoother surface is thus obtained. Boxwood tools charged with still finer diamond powder are next used, and after these copper tools charged with rotten stone and water. The harder gems, excepting the diamond, which is engraved with the greatest difficulty, are better adapted for this process than those of softer quality. The latter are liable to hold the diamond powder and cause it to wear out the tools; they do not when finished present such smooth and highly polished surfaces as the harder stones. The amethyst is considered as soft a stone as can be cut very smoothly. Car-nelian and bloodstone are of close texture, and admit of excellent work: the ruby cuts slowly, but small pieces are apt to flake off. The sapphire is firm and close; it cuts slowly, but presents beautifully smooth surfaces.-Artificial Gems. The great value attached to precious stones led at an early period to successful attempts to imitate them.

The Egyptians possessed the art of coloring glass, and among their mixtures they produced excellent imitations of the most beautiful gems, so that, as Pliny states, it was difficult to distinguish the false from the real. Their artificial emeralds, sapphires, and hyacinths are spoken of by various ancient authors. Some of the first named were of such gigantic size that they were used in the construction of statues, as of that of Serapis in the Egyptian labyrinth, 13 1/2 ft. in height. Another presented by the king of Babylon to an Egyptian Pharaoh was 6 ft. long and 4 1/2 broad; and an obelisk in the temple of Jupiter 6O ft. high and 6 ft. broad was composed of four artificial emeralds. These were very extraordinary productions if made only of pieces of colored glass. Seneca also makes mention of one in his time who manufactured artificial emeralds. Beck-mann states that in some ancient collections at Rome are pieces of colored glass, which were once used as jewels. In the Museum Victorium are seen a chrysolite and emerald of faultless execution.

In the 17th century the discovery of the preparation of gold and bin-oxide of tin, called purple of Cassius, afforded the means of giving a ruby-red color to glass, and artificial rubies were then first made, especially by one John Kunkel, afterward Lowen-stiern, inspector in 1679 of the glass houses in Potsdam. In modern times the art has been wonderfully perfected by the French, chiefly through the genius of M. Donault-Wieland. A glass called strass, of great lustre and perfectly transparent, of which the ingredients are given in the article Glass, is prepared as the basis of the composition. It resembles the diamond in high refractive power as in its other qualities, except hardness. That it may be free from color its ingredients must be absolutely pure; and care must be exercised in selecting crucibles least likely to impart color to the fused mixture. Artificial diamonds are made from pure strass, which is cut directly into brilliants and roses, without the addition of other matter. Other gems are imitated by melting and mixing it with substances having a metallic base, generally oxides, which communicate the various colors.-The researches and experiments of M. Ebelmen are of a higher order of art.

He conceived the idea in 1847 of forming various mixtures with some ingredients volatile at very high heat. By the dispersion of these in the furnace the other ingredients should crystallize, as substances held in solution in liquids crystallize when these are evaporated. He thus proposed to produce the ruby, corundum or sapphire, and other precious stones. The volatile solvents or fluxes he employed were principally boracic acid and borax. The spinelle ruby, among the first minerals he imitated, was obtained by mixing together 80 parts of magnesia, 25 of alumina, 1 of chlorate of potash, and 35 of boracic acid, and subjecting 500 grammes (7,716 grains) of the compound to high temperature in the muffle of a furnace for eight days. The crystals measured 0.197 inch on a side. Chrysoberyl was produced in crystals with faces of 0.24 inch, perfectly transparent, and scratching topaz from a mixture of alumina 12 grammes, glucine 3.5, carbonate of lime 10, and fused boracic acid 14 grammes. The object of the lime was to form a fusible borate for holding the other ingredients in a condition favorable for crystallization.

Chrysolite in well defined crystals resulted from silica 4.5, magnesia 6.15, and boracic acid 6. Transparent crystals of pure alumina, which are sapphire or corundum, and which presented the same hardness and specific gravity with this mineral, were a product of alumina thus fused with 3 or 4 parts of borax, or of 10 parts of alumina with 4 of silica and 16 of borax. M. Ebelmen employed also as fluxes the salts of phosphorus and the carbonates of potash and of soda, all which are volatile at high temperatures, and by means of these solvents reproduced many other minerals, as he announced to the academy in 1851. His death shortly after terminated his interesting researches; but they have been successfully continued by Deville, Eisner, Manross, and others, and numerous artificial minerals have been prepared.-The great establishment of M. Bourguignon in Paris was at one time the most famous manufactory of artificial gems in the world. About 100 workmen, besides many women and girls, were constantly employed in preparing and fusing the mixtures, cutting and polishing the stones, and lining the imitation pearls with fish scales and wax. The sand used to furnish the silica is from the forest of Fontainebleau; and its quality is so highly esteemed that much is exported for similar use elsewhere.

The gems are such perfect imitations that they can be distinguished from genuine stones only by the closest scrutiny of those experienced in such matters. The great hardness of the natural stones it is found most difficult to imitate, and there is a want of permanency in the qualities of most of the imitations, which at last causes their true character to appear.-See A Popular Treatise on Gems," by Dr. L. Feuch-twanger (New York, 1859; revised ed., 1867);"

The Natural History of Gems," by C. W. King (London, 1867); Diamonds and Precious Stones," by Harry Emanuel (London, 1867); and "Diamonds and Precious Stones," translated from the French of Louis Dieulafait by F. Sanford (New York, 1874).