IMITATION AND ENHANCEMENT

                           

  IMITATION gems have the appearance Lot their natural counterparts, but their physical properties are different. They are made to deceive. Man-made materials, such as glass and synthetic spinel, have been used to imitate many different also be modified to resemble more valuable gems. It is possible to enhance authentic gemstones by hiding cracks and flaws, or by heat-treating or irradiating to improve their color. 

GLASS IMITATIONS 

Glass has been used for centuries to imitate gemstones. It can be made either transparent or opaque, in almost any color, and, like many gems, has a vitreous lustre. At first sight, therefore, it may easily be mistaken for the real thing. However, it can usually be detected by its warmer feel, and by the evidence of wear and tear that results from its greater softness. Chipped facets and internal swirls and bubbles are common. In addition, unlike most of the gems it imitates, glass is singly refractive.

Opal Imitations

Gemmologists call the flashes of color in opal its "play of color", or iridescence. It is caused by the interference of light from the minute spheres of silica gel that make up the gem. This structure is imitated to great effect in opals made by the French manufacturer, Gilson, although the difference can be seen in the mosaic-like margins of the patches of color. There are various other opal imitations, including stones made of polystyrene latex, or of different pieces assembled as one. In an opal “doublet" (two pieces) the top is natural precious opal, but the base is common (potch) opal, glass, or chalcedony. A "triplet" (three pieces) has an additional protective dome of rock crystal.

SLOCUM STONES 

The American John Slocum developed imitation opals with a convincing play of color, but they lack the silky, flat color patches of genuine opal, and the structure looks crumpled when magnified.

GARNET-TOPPED DOUBLET

 One of the most common "composite" stones (stones made of more than one piece) is the garnet-topped doublet, or GTD. A thin section of natural garnet is cemented to a colored glass base, which gives the GTD its apparent color. The deception is most easily seen at the junction of the two layers, which may be obvious. 

DIAMOND IMITATIONS

 Many natural materials have been used to imitate diamond, but zircon is the most convincing. Synthetic imitations are popular, but each has its faults (right). Imitations can usually be detected by testing the heat conductivity of the stone.

HEAT TREATMENT 

Heating may enhance or change the color or clarity of some gems. Techniques range from throwing gems in a fire to "cook", to use of sophisticated equipment. The outcome is certain for some gems (like aquamarine, changing from green to blue), but less so for others.

STAINING 

Stains, dyes, or chemicals can alter the appearance of a gem, coating just the surface, or changing the whole specimen. For staining to be effective, a stone must be porous or contain cracks and flaws through which the color can enter. Porous white how lite, for ex- ample, can be stained to imitate turquoise.

OILING

 Oils may enhance a gem's color and disguise fissures and blemishes. It is common to oil emeralds in order to fill their natural cracks and flaws.

Synthetic Gemstone

                                                          SYNTHETIC GEMSTONES 

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.