The discovery of the first successful method of producing synthetic gems was a momentous event, both in the jewelry industry and scientifically. In a sense, it is miraculous that manmade apparatus can produce synthetic gemstones that duplicate not only the chemical formula but exactly the same atomic structure of their natural counterparts. Gem crystals that in nature required perhaps thousand of years to grow to maturity, only to remain undiscovered for more millions of years, are now made artificially in but a few hours or a few months.
This assignment is the first of a series dealing with the various gem substitute; subsequent assignments will consider other synthetics, as well as imitations, doublets, bonded stones, coated stones, etc. Synthetic corundum is presented first because its successful synthesis represents the culmination of man's age old dream of reproducing the precious stones. By discussing it first, we have an excellent survey of the general filed of mineral synthesis and of the numerous experiments that led ultimately to the two most widely used methods by which gem minerals are produced artificially today.
Methods of Synthesis
There was a time when only the chemically simple minerals and those that had a strong tendency to crystallize were within the realm of the readily synthesized. It was not until near the middle of the 19th century that the first awkward but ingenious attempts at gem synthesis were successful. For the last decade, the interest in materials useful in laser technology has led many of the big companies in the country to turn their crystallographers toward the synthesis of many materials which could be "doped" with minute impurities to give just the fluorescence needed. As a result, many rather unusual materiel is have been synthesized. For example, the diamond imitation, yttrium aluminum oxide, with a garnet structure has been made, in this case by the Czochralski method, in which a rod is pulled slowly from a melt. As the material leaves the molten mass, it crystallizes on the end of the rod.
Many minerals have been considered as possibilities for synthesis, but among the most difficult to synthesize has been diamond. In order to synthesize diamond, enormous temperatures and pressures are necessary. In 1955, General Electric was able to synthesize tiny diamonds. Quantity synthesis was established, however several thousand crystals were required to produce one carat. These small crystals were found to be more practical for grinding purposes than the crushed bort that had been used for years before. Crushed bort is still very important in industry, but the synthetic diamond has kept the price of natural grit much lower than it would have been otherwise.
In 1970, General Electric announced the production of jewelry quality synthetic diamonds in sizes up to slightly over one carat in the rough. This requires huge equipment in order to reach and maintain over a week the tremendous temperatures and pressures necessary to form diamonds. Thus, the potential for a gem synthetic diamonds at competitive prices to the natural still seems quite a distance in the future.
Fortunately, it is possible to identify these synthetic diamonds as they have appeared to date. General Electric seems to be aiming mainly at properties in a diamond that will make it most useful in industry, and so far this makes it possible to identify them rather readily.
There are three major methods used today to make synthetic gem minerals commercially. In the first, the flame-fusion process, powdered oxide are melted in a furnace by a high-temperature flume and accumulated in hardened form as cooling and crystallization take place. By this technique, corundum and a number of other gem minerals are made, including rutile, strontium titanate and spinel, as well as the non gem minerals cadmium tungstate and calcium tungstate (synthetic scheelite). By the second method, a hydrothermal process, quartz, opal, beryl (emerald) corundum and fluorite are crystallized from solution under conditions of high temperature and pressure, by dissolving fragments of the mineral and re-depositing it as transparent, gem-quality crystals. In laboratory experiments, most of the important gem minerals have been produced by crystallization from solution (hydrothermal methods); however, only those mentioned above are produced on a commercial scale. The third method is termed flux growth, and this is a means by which under high temperature and pressure conditions, materials are dissolved in a melt in which they are more readily soluble than in water. A modification of the melt-growth technique is the Czochralski or crystal-pulling technique. It is by this method that Chatham and Gilson have made their synthetic emeralds, and it is also the method by which Chatham and Kashan are making synthetic rubies. Synthetic alexandrites and sapphires are also flux grown.
In addition to the three methods described earlier, there are several others that are used, experimentally at least, for mineral synthesis. One would be the formation of minerals by direct passage from vapor to a solid state without first becoming a liquid called vapor growth or deposition. It was claimed by some of the early experimenters in diamond synthesis that tiny diamond crystals were produced by the solidification of carbon from a gaseous state, but this was never proved. DuPont makes tiny industrial diamonds by exploding charges in very strong-walled sealed containers with carbon in a gaseous state. They produce tiny synthetic diamonds that have a structure that makes them very useful for grinding purposes in industry. Another method that could be used, and one that may be used in the figure for some types of gem synthesis, is the precipitation of a substance from the solution in which it is dissolved by setting it free with chemical action, in other words, two or more chemicals are dissolved and a new material is formed by chemical action of ions of the dissolved materials.