By far the simplest in composition of all the precious stones is diamond, which is pure crystallized carbon; but its manufacture is attended by well-nigh insuperable difficulties. If carbon be heated in air, it burns at a temperature well below its melting point; moreover, unless an enormously high pressure is simultaneously applied, the product is the other form of crystallized carbon, namely, the comparatively worthless graphite. Moissan’s interesting course of experiments were in some degree successful, but the tiny diamonds were worthless as jewels, and the expense involved in their manufacture was out of all proportion to any possible commercial value they might have. Next to diamond the simplest substances among precious stones are quartz (crystallized silica) and corundum (crystallized alumina). The crystallization of silica has been effected in several ways, but the value in jewellery of quartz, even of the violet variety, amethyst, is not such as to warrant its manufacture on a commercial scale. Corundum, on the other hand, is held in high esteem; rubies and sapphires, of good colour and free from flaws, have always commanded good prices. The question of their production by artificial means has therefore more than academic interest. Ever since the year 1837, when Gaudin produced a few tiny flakes, French experimenters have steadily prosecuted their researches in the crystallization of corundum. Frémy and Feil, in 1877, were the first to meet with much success. A portion of one of their crucibles lined with glistening ruby flakes is exhibited in the British Museum (Natural History). [Illustration: FIG. 53.—Verneuil’s Inverted Blowpipe.] In 1885 the jewellery market was completely taken by surprise by the appearance of red stones, emanating, so it is alleged, from Geneva; having the physical characters of genuine rubies, they were accepted as, and commanded the prices of, the natural stones. It was eventually discovered that they had resulted from the fusion of a number of fragments of natural rubies in the oxy-hydrogen flame. The original colour was driven off at that high temperature, but was revived by the previous addition of a little bichromate of potassium. Owing to the inequalities of growth, the cracks due to rapid cooling, the inclusion of air-bubbles, often so numerous as to cause a cloudy appearance, and, above all, the unnatural colour, these reconstructed stones, as they are termed, were far from satisfactory, but yet they marked such an advance on anything that had been accomplished before that for some time no suspicion was aroused as to their being other than natural stones.