  Because of their rarity and relatively high cost, the number of real gems used throughout recorded times must be insignificant compared to the number of gem substitutes used. There are records of glass and ceramic imitations of gems as early as 3000 B.C. Certainly, the world gem markets today are flooded with man-made gems. There even has been developed a laboratory process for growing a coating of synthetic emerald on the surface of a faceted stone of natural colorless beryl. The recut gem looks like a natural emerald, and it has natural inclusions that totally synthetic emeralds lack. In general, gem substitutes can be classified as imitation stones, assembled stones, reconstructed and altered stones, and synthetic stones. IMITATION STONESAny material will serve as an imitation of a natural gem as long as it resembles the real thing under casual examination. Because of the great variety in types and colors available, glass and plastics are the most commonly used materials for making imitation gems. Almost every gem has been simulated effectively. The substitutes offer no difficulty of identification to the expert, but many are deceptive to the layman. ASSEMBLED STONESIt has been the practice for centuries to build up gemstones by fusing or cementing a shaped piece of natural gemstone to another piece, or other pieces, of inferior or artificial material. A colorless common beryl crown cemented to a pavilion of green glass produces an emerald doublet—part natural, part artificial—of good color and high durability. A thin piece of beautifully colored opal cemented to a base of inferior opal provides an assembled stone that looks like a thick piece of high-quality opal. Triplets, and even stones in which there are pockets of colored liquids or metal foil between the shaped pieces, are known. Usually, assembled stones are easily detected, since the joint will show under magnification, but sometimes they are mounted in settings that obscure the joint, and detection is more difficult. Assembled imitation gemstones. If it were measured on its natural ruby table, the assembled stone shown at top would have all the characteristics of a large ruby, including refractive index. The color of the quartz and glass combination (middle) depends on the color of the liquid in the cavity. Since emerald is green beryl, an inexpensive colorless beryl sandwich of green glass (bottom) would appear to be an expensive emerald. The joints of assembled stones often are hidden in the jewelry mountings. RECONSTRUCTED AND ALTERED STONESRuby fragments may be heated at high temperature to partially melt them into a large mass that can be cut into a more valuable stone. Ruby is the only stone that can be successfully reconstituted in this way, but there are many other ways of tampering with natural stones to make them more desirable. Sometimes natural stones are backed with foil or a metallic coating to enhance their color, to provide brilliance, or to produce a star effect. It is said that in an inventory of the Russian crown jewels by the Soviet Government, the ruby-colored Paul the First Diamond was discovered to be a pale pink diamond backed by red foil. Today, some diamonds are coated on the back with a blue film to improve their color. Aquamarine, when pale greenish blue, may be heated in order to deepen the blue color, and poorly colored amethyst may be heated to produce a beautiful yellow-brown quartz, called citrine, that often is misrepresented as topaz. By strong heating, the brown and reddish brown colors of zircon can be changed to blue or colorless, both of which states are unknown in natural zircon. Dyes, plastics, and oils are used to impregnate porous gems such as turquoise and variscite, and even jade. Off-color diamonds, when exposed to strong atomic radiation, can be changed to attractive green, brown, and yellow colors, causing them to resemble higher-priced fancies. In the constant search for something new, gem suppliers sometimes introduce into gemstones colors that are not always an improvement. For example, the beautiful purple of some amethyst can be converted, by heat treatment, to a peculiar green. Such an altered stone is marketed as greened amethyst. All of this tampering with gemstones complicates the problem of identification, so it is a matter of serious concern to the gem trade. SYNTHETIC STONESFor over 200 years mineralogists have been devising techniques for producing synthetic minerals in the laboratory, and attempts have been made, sometimes with considerable success, to apply these techniques to the production of synthetic gemstones. To qualify as a synthetic gemstone the man-made product must be identical chemically and structurally with its natural counterpart. Sapphire, ruby, spinel, emerald, and rutile in gem quality have been brought to commercial production. Two of the basic techniques used in producing synthetic gems are the flame-fusion and the hydrothermal processes. The Verneuil furnace, for making synthetic gem rough. A mixture of hydrogen (H) and oxygen (O) burns almost explosively, heating the fusion chamber (F) to high temperatures. For example, powdered aluminum oxide and coloring agents are sifted down from hopper (A) to the fusion chamber and form a cylindrical boule (B) on an adjustable stand (C). In the flame-fusion process—invented in 1904 by the French chemist Verneuil—powdered aluminum oxide, containing coloring agents, is sieved down through the flame of a vertical blowtorch furnace. As it passes through the flame, the powder melts and accumulates as drops on an adjustable stand just below the flame, where it forms a single crystal boule of the synthetic rough. In a few hours a boule of several hundred carats can be formed. When such furnaces are operated in banks of several hundred units, the commercial production of In the hydrothermal process, which differs greatly from Verneuil’s flame-fusion process, crystals are grown from solutions of the raw materials that have been subjected to varying conditions of very high pressure and temperature. Some of the quartz used for electronics purposes also is manufactured in this way. Since chemical composition and crystal structure are the basic characteristics by which a gemstone is identified, and these characteristics are identical in both the manufactured stone and its natural counterpart, the synthetic gemstones offer a very serious challenge to those concerned with gem identification. |
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