Imitation Gems

Imitation gems shave the appearance of their natural counterparts. However, 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 gems. But natural stones can also be modified to resemble more valuable gems. It is possible to enhance authentic gemstones by hiding flaws and cracks, or by heat-treating or irradiated them to improve their color.

Table of Contents

Glass Imitation Gems

Glass has been used for centuries to imitate gemstones. It can be made either transparent or opaque, in almost any color. Like many gems, it has a vitreous luster. 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.

Glass imitations often have pronounced inclusions. Chipped facets and internal bubbles and swirls are common. In addition, unlike most of the gems it imitates, glass is singly refractive.

Opal Imitations

Gemologists call the flashes of color in opal its ‘play of color’ or iridescence. It is due to 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. 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, chalcedony, or glass. A ‘triple’ (three pieces) has an additional protective dome of rock crystal.

Diamond Imitation Gems

Many natural materials have been used to imitate a diamond, but zircon is the most convincing. Synthetic imitations are popular, but each has its faults. Imitations can usually be detected by testing the heat conductivity of the stone. Yttrium aluminum garnet (YAG), cubic zirconia, and strontium titanate are imitations of diamonds.

Slocum Stones

The American John Slocum developed imitation opals with a convincing play of color. However, they lack silky, flat color patches of genuine opal. Also, 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. That gives the GTD is apparent color. The deception is most easily seen at the junction of the two layers, which may be obvious.

Heat Treatment of Gemstones

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

Irradiation

Gems may change color if exposed to radiation. This may come from radioactive elements within the Earth’s crust, or from natural sources. Natural radiation may take millions of years to have an effect, while artificial irradiation may take only a few hours to change a gem’s color. In some cases, a gem will revert to its original color or may fade with time. Many changes can be reversed or modified by heat treatment.

Staining

Stain, 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 howlite, for example, 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.