Often, one gemstone is substituted for another of similar appearance because of price or availability. Such gem substitutes may be natural or manmade. Manmade gem materials may have nearly identical natural counterparts, in which case they are referred to as synthetic stones, or they may have no similar naturally occurring counterpart. Here are some materials made by man that are commonly used as substitutes for natural stones. Some are quite easy to identify; some are quite difficult. Buyer beware! While the major producers of synthetic gems actively support the full disclosure of their stones' laboratory origin, and the jewelry industry officially insists on such full disclosure, there are still too many jewelers and gem dealers who are not so forthcoming with this information. Federal Trade Commission rules and the policies of jewelry industry organizations are actively demanding full disclosure of origin and treatments. When you buy gemstones, you should specifically ask about the origin of the stones and whether they have been treated in any way other than cutting and polishing.
Synthetic corundum, in the form of synthetic ruby, was the first gemstone reproduced by artificial techniques. Auguste Victor Louis Verneuil invented the flame-fusion technique (now know as the Verneuil method) of melting aluminum oxide and allowing it to recrystallize in a cone-shaped boule, the color of which can be controlled by the addition of chemical additives. This material is abundantly and inexpensively created and used routinely in class rings, birthstone rings, etc. Such material is rather easy to distinguish from natural material by the presence of curved growth striations and spherical gas bubbles or by the Plato method, in which repeated twinning lines appear when the material is immersed in high R.I. (refractive index) liquid and examined under magnification between crossed polarized filters.
Other methods of growing synthetic corundum, such as flux-grown, Czochralski pulled, and hydrothermal methods, produce more realistic imitations, which are primarily identified by characteristic inclusions. They are much more expensive than the Verneuil synthetics but considerably cheaper than the rubies and sapphires that they imitate.
Synthetic spinel is produced by the Verneuil method and is used most often to imitate stones other than spinel, such as blue sapphire or aquamarine. It is easily identified by its R.I., spherical gas bubbles, and characteristic strain lines seen between polarizing filters. It is quite inexpensive and widely used in inexpensive jewelry.
Synthetic diamond is produced under high pressure and temperature and, to date, has not been produced at low enough costs to be used as a natural diamond substitute. However, there are indications that we may begin to see gem quality synthetic diamonds within the next few years. Synthetic diamond is used very extensively (two-thirds of the world production) for industrial purposes as abrasive diamond grit.
Synthetic emerald is produced in some abundance by flux-growth, hydrothermal, and Lechleitner overgrowth methods and sold under trade names such as Gilson and Chatham. It can sometimes be difficult to distinguish from its natural counterpart, but the presence of characteristic inclusions is usually the giveaway.
Synthetic opal, coral, lapis lazuli, jade, turquoise, and quartz have only in the past few years appeared on the market in fairly good imitation of the natural material. Microscopic examination usually reveals their identity. Recently, synthetic amethyst has become very abundant, mixed into parcels of natural stones; it can usually be detected by the absence of twinning, but recent improvements in synthesis are making it more difficult to detect, and the relatively low cost of both natural and synthetic amethyst makes it prohibitively expensive to do routine testing of large quantities of amethyst.
Synthetic alexandrite has come into the market in greater abundance in recent years and is produced by the Czochralski or "pulled" method and by the "floating zone" or "floating point" method. It is more realistic and more expensive than the cheap color changing Verneuil corundum that has usually been used as a substitute for alexandrite.
Glass, sometimes called "paste," by virtue of its wide range of color and low cost, has been used for centuries to imitate natural gemstones. It is easy to identify by its R.I., spherical gas bubbles, swirled growth pattens, and characteristic strain lines seen between crossed polarizing filters.
Cubic zirconia (CZ) is the most abundant diamond imitation and is produced very inexpensively. It has no close natural counterpart. CZ is fairly convincing as a diamond substitute but is not difficult for a trained eye to detect. Thermal "diamond probes" provide quick and easy separation from diamond. CZ is now produced in a wide variety of colors and often sold under a variety of company tradenames.
Moissanite in mid-1998 began to make a huge splash in the gem world as the latest and greatest diamond simulant. This synthetic silicon carbide crystal was discovered by a materials scientist at North Carolina State University and is being developed by Charles & Colvard (formerly known as C3) and Cree Research. Its natural counterpart has been found only in meteorites and is deep green, but the synthetic moissanite is near colorless and will probably continue fool quite a few jewelers and jewelry buyers. Its high refractive index and extreme hardness (second only to diamond) make it an effective diamond simulant, and the popular diamond probes that test thermal conductivity fail to distinguish it from diamond. Charles & Colvard is also marketing a detector that can distinguish between moissanite and diamond. Another way to identify it is by its double refraction -- look through the stone from several different angles and you should see double images of facet lines or objects viewed through the stone. If you look directly through the table, you're not likely to see the doubling, though, since it's oriented on the optic axis. Keep your eyes out for this one!
Manmade garnets such as YAG (yttrium aluminum garnet) and GGG (gallium gadolinium garnet) and other materials such as synthetic rutile and strontium titanate preceded cubic zirconia as diamond simulants but were never very convincing in that role. With the advent of CZ and, more recently, moissanite, their role in the market has declined considerably.