GEMS, ARTIFICIAL, denote precious stones artificially made and having the chemical, physical and optical properties of natural gems. Of the precious stones, the diamond, emerald, ruby and sapphire have all been successfully synthesized. Only the ruby and the sapphire, however, have outgrown the laboratory stage of the production and represent the principal products of the synthetic precious stone or artificial gem industry. Artificial amethysts, garnets, tourmalines, etc., have not appealed to the industry because they are not valuable enough as natural products.
The pearl is almost altogether of animal origin. Only the dia mond, emerald, ruby and sapphire will be treated here. The beauty of natural precious stones is due to their composition and cutting. Permanence of beauty is due to the hardness of the stones and to the fact that they are chemically inert. Extra neous conditions, such as custom and rareness may also affect their value.
The Diamond.—The transformation of the diamond to graphite or carbon has been accomplished frequently. The process has also been reversed and some few diamonds have been produced by various investigators. The work of Moissan is an interesting instance. He developed his experiments from the analytical work of Friedel. The Devil's canyon in Arizona was once littered with meteorites. Some of these were analyzed by Friedel. Tiny dia monds were found embedded in the mass of iron. Moissan tried to reproduce the conditions of the fiery meteor. In an electric furnace he placed a carbon crucible containing pure iron and carbon. The carbon dissolved in the molten iron until a saturated solution was formed, and while the material was at white heat he plunged it in a bath of molten lead. The sudden cooling caused tremendous internal pressure and the liquid carbon was crystal lized into small diamonds. One of these, weighing about 6 mg. a carat), when burned in oxygen produced about 23 mg. of carbon dioxide. Theoretically, 22 mg. should have been produced. Up to 1928 synthetic diamonds had not been commercially produced although numerous experiments were in progress. The high dispersion of light gives to the diamond its so-called "play of fire." It reflects almost all the rays of light that strike its surface and give it the characteristic lustre known as adamantine.
The Emerald.—Chemically, the emerald is a metasilicate of aluminium and glucinum. Hautefeuille and Perry, 189o, dissolved the constituents of the gem in their relative proportions in a bath of dimolybdate of lithium and, keeping the bath at 800° C for 15 days, succeeded in crystallizing out tiny emerald crystals. A little chromic oxide was used to give the green colour. These crystals were perfect, but too small; the largest being only 2 mm. long by I mm. wide and 1 mm. thick. They were much more expensive than the natural product. Beyond this successful laboratory attempt, no successful means have been found up to 1928 for pro ducing the emerald on an industrial scale. Beryl, the natural metasilicate of glucinum and aluminium, is golden yellow in colour. The emerald is identical with it save its green colour. This difference is due to the presence of different impurities. Al most all the rays of light impinging upon the surface of an emerald enter it, and very few are reflected from its surface, very much as in the case of glass, and the emerald is not as optically dense as the diamond.
The Ruby.—The ruby was the first of the precious stones to be synthesized on a commercial scale. Chemically, it is the oxide of aluminium with a trace of chromic oxide, to which it owes its rich pigeon-blood colour. Corundum is often free from the oxide of chromium and we have either a colourless stone or, if some other oxides are present, we may have a blue stone—the sapphire, or shades of green, smoky tinges, etc. The effect of radium emanations on natural and synthetic rubies is shown in an experiment by F. Bordas. Using radium bromide of 1,800,000 activity he turned natural rubies into a brick red colour. The syn thetic stones were not affected. But the synthetic product is often ruined during manufacture by quantities of impurities that are so small they can scarcely be detected by analytical methods.
An interesting attempt to crystallize aluminium oxide out of a molten bath was that of Fremy and Hautefeuille. The oxides of lead, aluminium and chromium were fused in a large crucible for about seven to eight days in a furnace used for glass making. Masses of rubies from 3o to 4o kg. were sometimes obtained, but among these not a ruby was found that was thick enough to be of any value as a gem. One of the crystallographic axes developed more than another, and only thin, laminated crystals were pro duced. Attempts to condense alumina vapour produced similarly imperfect gems. Fremy and Verneuil carried on a series of ex periments with ordinary sand crucibles for this purpose. It was found that in the presence of vapour of potassium carbonate amorphous alumina would be changed into corundum. Sand cruci bles were filled with alumina, potassium carbonate and chromic oxide; mixed with fine charcoal, packed around a core composed of alumina and calcium fluoride and gently heated. The charcoal escaped as carbon dioxide and made the mass porous. Af ter eight days the largest stone produced was 4 to 5 mm. long and about I mm. thick, weighing 6o mg. or about of a carat. These not only were too thin to be cut into gems, but lacked the beauty of the natural stone which appears to be made up of a series of layers. The light coming through these layers gives to the ruby its wealth of colour. Rubies were made by a priest near Geneva, by fusing chips of natural rubies into stones. Verneuil succeeded in fusing the oxide of aluminium and developed the process to a point where rubies were made that were large in size, of good colour and at a speed sufficient to make an industry possible. One work man can operate about ten furnaces producing 30 or more carats per hour per furnace.
Rubies are made commercially by fusing alumina in the oxy hydrogen flame and permitting the molten mass to solidify in the cooler zones of the flame. A modified inverted oxy-hydrogen torch of Deville is used in modern practice. It produces a flame of many zones varying in temperature from 1,900° to 2,400° C. The torch consists of two concentric tubes : the inner tube carries the oxygen, extends about a foot beyond the outer and has a cylindrical shaped top. In the lower portion of the torch, the outer or hydrogen-carrying tube extends an inch or two beyond the oxygen tube. The ruby-forming powder is placed in a sieve bottom box which, in turn, is screwed into the cylindrical top of the oxygen tube. When the torch is lit a hammer is caused to knock periodically on the top of this box, and particles of alumina are thus blown into the flame. In the beginning of the process, the flame is comparatively cold and just heats a rod that is placed to catch this powder. As the powder continues to fall on this rod it forms a pyramid of fritted alumina. The heat is gradually increased until the top of this pyramid becomes molten and a tiny stalk or "pin-head" begins to grow. At this stage the flame is made still hotter and the powder falls in molten drops upon this pin-head. Each succeeding drop falls upon a larger base until an unflawed pear-shaped, or so-called ruby boule, is produced.
This boule is one single crystal with the optical axes directly perpendicular to one another. When the stem of this boule is broken the stone breaks in two. Pure alumina is essential for this process. The presence of even o.0005 of i ;o of a certain impurity jeopardize the industry, as this amount is sufficient absolutely to discolour the ruby, and produce a brick-red instead of the pigeon blood stone. This impurity was found by Levin to be magnesium oxide Mg.O. The stone cut from a boule is physically and chem ically identical to the natural ruby. There is perhaps but one method of telling the synthetic from the natural stone. In the natural stones the imperfections have flat bounding sides and are so-called negative crystals; the imperfections in the synthetic stones have round surfaces and are simply air bubbles which in many cases can be detected only by a powerful magnifying glass. Both the natural and the synthetic stones are made up of a series of successive layers; the synthetic stone having curved layers, the natural product flat, parallel layers.
The higher oxide of titanium, is the stable oxide in molten corundum and gives to the corundum a lavender colour. Pinkish stones and not blue stones were first obtained in the fusion of the iron oxide and titanium oxide with corundum. A greater quantity of iron oxide produced the necessary reduction of the titanium oxide and the true blue colour of the natural sapphire was finally obtained. The reaction is delicately balanced and prolonged heat ing will cause the blue colour to change back to pinkish. This explains why natural sapphire chips lose their colour when fused. The blue corundum boule is identical with the natural stone and they are distinguished only with great difficulty by experts. After fusion only part of the iron and titanium oxides are left. A. J. Moses found only traces of and about o• I of 1 % of The analysis of the natural products extant before the syn thesis of the sapphire did not show the presence of titanium oxide. The synthetic ruby or sapphire like the unpolished natural stones require the lapidary's art to bring out their intrinsic qual ities. About 20,000,00o carats of rubies and 12,000,00o carats of sapphires are produced annually and the demand is growing.
(I. H. L.)