Artificial Gems

The term artificial gem is used to describe either an imitation of a natural gemstone or a synthetic gem that is chemically identical to naturally occurring gems.

Imitation Gem

Such a gem may be made of flint glass, often silvered on the back to increase the brilliance. Since World War II, colored plastics have replaced glass, especially in costume jewelry. Plastics are cheaper, more easily molded, and lighter in weight.

During the 19th century, artificial pearls were made by blowing hollow beads of glass and pouring into them a mixture of liquid ammonia and the white matter from the scales of fish such as the bleak, roach, or dace. A much better type of artificial pearl, the indestructible bead, was introduced shortly after 1900. The bead is made of solid glass with only a narrow hole for the thread. Pearl essence, consisting of the crushed scales of certain herring, is applied to the outside of the glass and covered with a transparent, colorless lacquer.

The most successful imitation of a diamond is strontium titanate, made by a flame-fusion technique. Its index of refraction is almost identical to that of a diamond, and it has a higher dispersion. Thus, it has the brilliance and greater fire than the diamond. It scratches easily, however. A harder material simulating the diamond is rutile, or titanium oxide.

Synthetic Gems

The term is limited by the U.S. Federal Trade Commission to manufactured materials that duplicate a natural gemstone chemically, physically, and optically. Synthetic gems can be distinguished under a microscope because they are more perfect than natural gemstones and contain no irregularities.

Synthetic diamonds were first made by the General Electric Company in the U.S. in 1955. In their process, carbonaceous compounds are subjected to pressures of 56 metric tons per sq cm (360 metric tons per sq in) at temperatures of 2760° C (5000° F). The diamonds thus produced are suitable only for industrial use.

In the late 1960s a method was developed for “growing” diamonds by heating a diamond particle to a high temperature and subjecting it to methane gas. The gas decomposes into carbon atoms, which adhere to the diamond crystal. The crystal structure of the enlarged diamond is identical to that of a natural diamond. Diamonds of about 1 carat (200 mg or 0.007 oz) have been produced by this method, but their cost is still considerably higher than that of naturally occurring diamonds.

Sapphires are made in an apparatus resembling an oxyhydrogen torch. The flame is directed into a fireclay support inside an insulated chamber. The oxygen gas carries finely powdered pure aluminum oxide into the flame, and the powder fuses into droplets, forming a cylindrical boule, or matrix, on the support. The size of the resulting sapphire is controlled by varying the gas flow, temperature, and amount of powder. Boules weighing up to 200 carats (40 g or 1.41 oz) can be produced by this technique. Perfect rubies and sapphires up to 50 carats (10 g or 0.353 oz) have been cut from such a boule.

Rubies are made by the same process by adding 5 to 6 percent chromium oxide to the aluminum oxide. Colors other than red are produced by adding different metallic oxides. Stars can be added to synthetic rubies or sapphires by adding an excess of titanium oxide to the aluminum oxide powder and heating to temperatures greater than 1000° C (greater than 1832° F). In gems made with this technique, synthetic stars appear sharper than naturally occurring stars.Emeralds, some of which are of gem quality, are synthesized by still-secret methods. They can be distinguished from natural emeralds by their red glow under ultraviolet light.