Synthetic gem materials

Author: John Milton
The need for a variety of high-quality crystals for a range of industrial applications (i.e. optics components, laser crystals and other uses), and the increased awareness of crystal growth mechanisms, have since combined to yield the production of a wide variety of crystals, some of which are synthetic counterparts of natural gemstones. A synthetic gem material has the similar chemical composition and crystal structure as a natural gem mineral. In contrast, a gem stimulant or imitation has the appearance of a natural gemstone, but has a different chemical composition, physical properties, and crystal structure. Both synthetic and stimulant gem materials almost always possess gemological properties that allow them to be distinguished from the corresponding natural gemstones.Current methods used for gem synthesis fall into two general categories: crystallization from a fluid of different composition (such as a flux or aqueous hydrothermal solution), and crystallization from a melt with roughly the same chemical composition as the crystal being grown. Synthetic diamonds, grown from a metallic flux at high temperatures and pressures, have evoked concern in the jewelry trade ever since the first production of gem quality crystals in the early 1970s. However, due to the growth conditions involved, as well as the great expense and restricted availability of the growth equipment used, the actual number of gem-quality synthetic diamonds continues to be limited. Those that are encountered in the jewelry trade are mainly brownish-yellow crystals, weighing 1 carat or less (1 carat = 0.2 g), which can yield faceted pieces of 0.5 carat or smaller in size. In contrast to synthetic diamonds, colorless or near colorless imitation materials are much more abundant.

Over the years, a number of natural gem minerals and synthetic materials have been used to imitate colorless diamond. Cubic zirconium oxide (cubic zirconium or CZ) is the most widespread stimulant because of its low cost and similar appearance to a polished diamond. It (and most other stimulants) can be readily distinguished from diamond on the basis of a difference in heat conduction, which can be detected with a simple gemological test instrument. Within the past two years, a new material, synthetic moissanite (silicon carbide), has been marketed for jewelry purposes. It has caused some serious identification problems among jewelry, because it cannot be distinguished from diamond by the heat conduction test mentioned earlier. However, synthetic moissanite displays optical features due to its anisotropic optical character (seen as an optical doubling when viewed with magnification) which allow it to be easily recognized by trained gemologists.

Among colored stones, the most important synthetics are synthetic corundum (sapphire and ruby), emerald, spinal and amethyst. As indicated they are produced by both solution and melt crystallization techniques. In the marketplace, flame-fusion and crystal-pulled synthetics are relatively less expensive and therefore are more abundant than the flux and hydrothermal synthetics. Solution-grown synthetic amethyst falls into the same less-expensive category because of the existing large-scale production facilities for synthetic quartz for use in the electronics industry. In contrast, hydrothermal- and flux-grown ruby, sapphire and emerald are considered as ‘luxury’ synthetics that command higher prices.