Gemstones are described by gemologists using technical specifications: chemical composition, crystal type (diamonds are cubic), species (rubies are the red variety of corundum and sapphires are any other color of corundum), refractive index, hardness, luster, etc. In addition, gemstones can be classified as to their “occurrence,” referring to where that particular gem is found naturally. Synthetic gems share all of these characteristics, except occurrence. Synthetic gems “occur” in a laboratory.
Several new types of synthetic and simulated gems have hit the market in the last couple of decades, but synthetic gems have been around for a very long time. The first synthetic ruby was produced by A. V. Verneuil in the late 1800s using a flame fusion method, a procedure that is still in use today.
The value of synthetic gems is twofold: low production cost, and the ability to control characteristics. Synthetic gems contain the same mineral properties of naturally existing gems, including the trace elements that cause color. They have the same brilliance and fire, and are often of better structure, since growth is controlled. The gems are cut to near ideal proportion, and to the naked eye are essentially flawless.
Creating synthetic gems is accomplished by one of three types of processes: melt growth, solution growth or extremely high pressure, high temperature (HPHT) growth. Verneuil used flame-fusion to produce the first ruby. Later, sapphire was grown with this technique as well. Flame-fusion produces a single crystal, called a boule, by pouring pure oxides (aluminum for ruby, sapphire) into a small furnace. Other oxides are added for color and to control the formation process.
The resulting melted material, or boule, solidifies on a rotating peg as it comes out of the furnace. A boule is characteristically a long cylinder with one rounded end, and one tapered end; 13-25 millimeters wide (diameter), 50-0100 millimeters long, weighing 75 to 250 carats. From this boule, gemstones are cut and faceted.
The Bridgman-Stockbarge solidification method, named for American P. W. Bridgman and German D. C. Stockbarge is a another melt method of creating synthetic gems, perfected between 1924 and 1936. This process is currently used to grow various non-gem crystals and a few metallic oxide crystals, namely sapphire. In this method, a crucible (a device made from a high temperature substance to hold a high temperature substance) is used, specially shaped into a cylindrical tube with one open end, the other capped by a pointed cone.
The crucible itself is filled with raw materials and lowered into the furnace. The pointed end cools first, creating, hopefully a single crystal, which then acts as base for the rest of the molten material, ostensibly forming one large crystal. Various sizes of crystals can be grown via the Bridgman-Stockbarge method, typically 50 millimeters by 15 millimeters. Large ones, however can exceed 890 millimeters, and weigh more than 1,000 carats.
To grow rubies, sapphires, garnet, and alexandrite the Czochralski pulled-growth method, developed in 1917, is used. Ingredient powders are melted in platinum, graphite or ceramic crucible. After attaching a seed crystal to the end, a rotating rod is lowered into the crucible, just barely touching the surface.
The rod is then withdrawn and the crystal grows as the seed “pulls” material from the molten ingredients, and cools. The surface tension of the air keeps the growing crystal in contact with the molten material, so it continues to grow until the melt is depleted. Crystals grown with this method can be very large and of very high purity, producing millions of carats annually.
Some synthetic gems pose unique growing problems. Certain materials are so either so reactive they can’t be melted in an un-reactive platinum crucible, or melt at higher temperatures than the crucible materials can withstand. For these synthetic gems, a skull melting system must be used. The "skull" is a hollow-walled copper cup. Water circulated through the copper “skull” to cool its inside wall. The cup is then filled with required ingredients and heated by radio frequency. Because the walls are cool, the material next to the wall doesn’t melt, and instead forms a crust, such that the molten material is being held within itself. As with other melt methods, once the material cools, crystals form.
Other growth methods for synthetic gems include solution techniques for emeralds, hydrothermal method for aquamarine and others. Quartz crystals are grown in a solution in autoclaves, where temperature and pressure are controlled to create the feed material called lascas in the hotter portion. Seed crystals are in the cooler portion upon which the lascas redeposits, forming synthetic quartz.
This process can take 30 to 60 days, and is also used to grow amethyst, citrine, or rock crystal. Turquoise, jade and lapis lazuli are grown via liquid-state reactions and phase transformations. Of all the methods available, the Verneuil flame-fusion, the Czochralski pull method, and the skull melting processes are the ones most often used for gem materials.
As mentioned previously, several types of synthetic gems have emerged in the past few years. Much of this growth in the due to recognition of synthetic gems for their own merits, not just as less expensive substitutes. In 1993, production value of synthetics and simulants was reported as $17.9 million. During the past five years that value has risen to $18.8 million annually.
Synthetic gems have the same composition as their naturally occurring counterparts, and are often of higher purity. Many believe, however, that since they are “grown” and aren’t “natural” that they aren’t real, that owning a synthetic gem is like owning a copy of a Rembrandt. Whether real, or not, synthetic gems are a beautiful and economical alternative worth considering.
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