As early as 1837 Gaudin made artificial rubies by heating ammonia, alumina, and potash by means of an oxy-hydrogen blowpipe; the intense heat volatilised the potash and alumina, afterwards producing crystals in rhombohedral forms identical with those of the natural stone, and having the same specific gravity and hardness. Methods of producing crystals of corundum, ruby, sapphire, etc., were discovered about 1858, but both these and Gaudin's processes had but little commercial value, the great expense precluding their adoption. Until quite recently, the only artificial gems known to commerce were coloured glass, and, in some cases, wax preparations backed with silver or a mercury amalgam. Now, however, the chemist can produce imitations that, in lustre and hardness, equal the real or found gems; perhaps " imitation " is not the correct word, as the composition of both manufactured and found stones is supposed to be the same. Sometimes it is quite impossible to distinguish between the two kinds of gems, although generally examination under the microscope discloses some difference. As seen through a microscope, natural rubies contain minute cracks which indicate the lines of cleavage; the artificial gem shows very minute bubbles or gas holes.

Analysis has proved that the sapphire is pure alumina, that is, oxide of aluminium (AI2O3). This is found in the form of a white powder fusible at high temperatures only. The colour of a sapphire is supposed to be due to the presence of chrome, and is dichroitic, that is, it varies with the point of observation; thus it is successfully imitated only with difficulty. M. Sidot, the French chemist, accidentally discovered a method of producing gems that possessed dichroitic properties. His method is to heat an iron pot to dark red and to place in it 4oz. of superphosphate of lime; this is brought to the same heat and stirred with an iron rod, being then converted to crystallised pyrophosphate, which, on being further heated, becomes a fluid resembling molten glass. It is supposed that in this state a part of the phosphoric acid is changed to a tribasic phosphate. The fused mass is stirred continuously until it is quite transparent and free from bubbles, when it is transferred to another pot and kept at a white heat for two hours, the stirring being kept up all the time. After standing for an hour, it is poured on to a metallic surface and allowed to cool slowly until as soft as putty, when it is put on plate glass.

When cold, a number of stones almost equal to the genuine sapphire may be cut from the plate. Another formula is: Smelt a mixture of 4 oz. of oxide of aluminium and 4oz. of red lead (Pb3O4), and stir in l0gr. of bichromate of potassium (K2Cr2O7) and 17gr. of oxide of cobaltum (CoO). When cold, stones may be cut that are as hard, if not quite so brilliant, as the genuine ones. The ruby, also, is oxide of aluminium coloured by chrome. Crystals of the rose-coloured ruby may be produced by melting together aluminium oxide and powdered silica, with the addition of fluoride of barium to form a flux, and then adding a trace of bichromate of potassium; 5001b. of these ingredients, after perhaps a week's fusion, will produce rubies of 5 or 0 carats which may vary much in colour, running through all the shades of bluish sapphire and rose to the deep colour of the so-called pigeon-blood ruby. Ordinary borax fused with a little chromium oxide for a week or so produces large ruby crystals; but 2001b. of ingredients may be required to obtain even two or three gems of any marketable value. One method of making artificial rubies is to smelt a mixture of 4oz. of oxide of aluminium and 4oz. of red lead, and add from 7 gr. to 10 gr. of bichromate of potassium.

Natural emeralds are a combination of the rare element beryllium or glucinum with silicon; chrome gives the colour. Beryllium is too expensive for use in producing imitations, so oxide of aluminium is used, 4 oz. of this being smelted with 4 oz. of red lead, to which from 8gr. to 12 gr. of uranate of sodium (Na2U2O7) have been added. Hautefeuille & Perry, the French chemists, produce some beautiful emerald crystals by fusing silica, alumina, glucina, and a trace of chromium oxide with acid molybdate of lithia. After a fusion of fifteen days some very small crystals, having all the mineralogical and physical characters of the natural emerald, may be obtained. The longer the fusion the larger are the crystals. Emeralds and other gems have been produced from gas retort refuse by a method discovered by Mr. Greville Williams, F.R.S., who modelled an emerald composed of from 67 to 68 per cent, of silica, 15 to 18 per cent, of alumina, 12 to 14 per cent, of glucina, and traces of magnesia, carbon, and carbonate of lime. The colour was an intense green, due, it is believed, to the presence of sesquioxide of chromium.

Imitations of the amethyst, topaz, etc., have been made very successfully by Donault Wieland, of Paris, whose method of preparing "Parisian diamonds" or " Alaska diamonds " is to smelt a mixture of 65 per cent, of pulverised crystal quartz, 20 per cent, of red lead, 8 per cent, of pure carbonate of potash, 5 per cent, of boric acid, and 2 per cent, of white arsenic. The brilliancy of the resultant stone depends principally on the purity of the red lead and of the carbonate of soda.