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IMITATION
Imitation gems have the appearance of 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 gems, but natural stones can 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 colour.
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GLASS IMITATIONS Glass has been used for centuries
to imitate gemstones. It can be made either transparent or opaque, in almost
any colour, 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. |
Glass Ruby |
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HEAT TREATMENT
Heating may enhance or change the colour 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. |
Yellow topaz heated to Blue |
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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 colour can enter. Porous white howlite, for example, can be
stained imitate turquoise. |
Stained howlite |
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IRRADIATION
Gems may change colour if exposed to radiation. This may
come from radioactive elements within the Earth's crust, or from artificial
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
colour. In some cases a gem will revert to its original colour, or may fade
with time. Many changes can be reversed or modified by heat treatment. |
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OILING
Oils may enhance a gem's colour and disguise fissures and
blemishes. It is common to oil emeralds in order to fill their natural
cracks and flaws.
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Oiled emerald |
| Natural Gemstones |
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