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 STONES Any 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 STONES It 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. [Illustration: 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 STONES Ruby 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.