SYNTHETIC GEMSTONES are made in laboratories or factories, not in rocks. They have virtually the same chemical composition and crystal structure as natural gemstones, so their optical and physical properties are very similar. However, they can usually be identified by the differences in their inclusions. Many gems have been synthesized in the laboratory, but only a few are produced commercially-generally for industrial and scientific purposes.
MAKING A SYNTHETIC
Man has tried to replicate gemstones for thousands of years, but it was not until the late 1800s that any substantial success was achieved. In 1877 French chemist Edmond Frémy grew the first gem-quality crystals of reasonable size (see bottom right), and then around 1900 August Verneuil devised his technique to manufacture ruby. With a few modifications, the Verneuil "flame- fusion" method is still in use today. The powdered ingredients are dropped into a furnace and melt as they fall through a flame hotter than 2,000°C (3,630°F), fusing into liquid drops. These drip on to a pedestal and crystallize. As the pedestal is withdrawn, a long, cylindrical crystal, which is known as a boule, forms.
FLU MELT TENIQUE
Pionred by the French chemise Edmond Frémy, the flux-melt technique is still used to make emeralds. The powdered ingredients are melted and fused in a solvent (flux) in a crucible. The material must be kept at a very high temperature for months, before being left to cool very slowly.
FLAME-FUSION CORUNDUM
Synthetic corundum manufactured by flame-fusion grows as a single mass called a "boule". It has the same inner structure as a natural crystal and can be cut to shape.
SHAPES AND COLORS
Because of the way they are made, synthetic gems may show subtle differences in shape and colour that help to distinguish them from their natural counterparts. For instance, corundum produced by flame-fusion has curved growth lines, rather than straight ones, because the ingredients have not mixed together fully. Some synthetic gems may also suffer from uneven colour distribution. Flame-fusion spinel is manufactured to imitate gems such as ruby, sapphire, aquamarine, blue zircon, tourmaline, peridot, and chrysoberyl.
DISTINCTIVE INCLUSIONS
Synthetic gems have different inclusions from natural gems, so often the best way to tell them apart is to examine them with a loupe (below) or a micro- scope. Synthetic inclusions may be typical of a process, or of a synthetic gem species. For instance, in Verneuil rubies, gas bubbles have well-defined outlines; in flux-melt emeralds (right), characteristic "veil" and "feather" patterns form.
GILSON GEMS
Lapis lazuli, turquoise, and coral produced by the French manufacturer, Gilson, are similar to their natural counterparts, but are not true synthetics because their optical and physical properties differ from the natural gems. Gilson lapis lazuli, for example, is more porous and has a lower specific gravity.
GILSON EMERALD INCLUSIONS
Synthetic emeralds from the French manu- facturer, Gilson, have characteristic veil-like inclusions. The gems are made from poor- quality material by a flux-melt method.
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