attractive object is rarely in the market – place for long before it is
copied or imitated. Gemsotnes have been copied for at least 6000 years
by a variety of materials which can be described as imitations, composite
stones or synthetics.
gems look similar to natural stones, but are usually very different in their
composition and optical and physical properties. They may be artificial
substances or natural minerals of similar colour to the desired gem. Glass
is a favourite stimulant used to imitate many different gemstones, because
it can be made in almost any colour and either moulded or cut to shape. Most
glass, however, is much softer than the gems that it imitates and becomes
badly chipped with wear. It may also contain bubbles (see below) and display
a distinctive treacly or swirly texture. Glass is singly refractive, with a
refractive index ranging between 1.5 and 1.7 - no singly refractive gem
minerals fall within this range.
widely used diamond stimulant of the last 20 years has been cubic zirconia
(CZ), which was originally produced for use in laser and electronics
research; more recently synthetic moissanite has come on the market. With
their high refractive indices and fire, both are difficult to detect by
visual tests alone. CZ can be distinguished from diamond by its lower
reflectivity and heat conductance,. Heat conductance is measured using a
the late 1990s, synthetic moissanite, a silicon carbide (SiC), is a new
synthetic gemstone with properties that are close to diamond than any other
imitation. With a hardness of over 9 on Mohs’ Scale, it is harder than ruby,
sapphire or any other gem, except diamond. However, there is one main
visible differences in that diamond is singly refractive and moissanite is
doubly refractive. Because of this, a doubling of the pavilion facets can be
seen on larger stones.
is difficult to recognize without the use of instruments when the stonesare
particularly small and set in jewellery. The RIs of moissanite are 2.64 and
2.691, dispersion is 0.104, birefringence is 0.0043 and specific gravity is
and enhanced Stones.
A number of
techniques are used to improve the colour and /or appearance of natural and
synthetic gemstones. The purpose is to increase their beauty,desirability
and salability. Probably the oldest method is that of heat – treating a
gemstone to improve or change its colour. The heating of carnelian has been
carried out in India for over 4000 years and oiling of emerald has been
known for over 2000 years.
As a result
of recent advances in technology, there are now many different techniques,
which use modern equipment such as lasers, and computer – controlled heating
and irradiating procedures. Lasers are used to drill holes into diamonds to
reach inclusions. These are then evaporated or removed using chemicals,
before the crack is filled. Some treatments are permanent, such as drilling,
while others may be relatively temporary; for example, stains and fillings
may leak, and some heated and irradiated stones may fade or revert to their
and sapphires are heat – treated to improve their colour. Sapphires
considered too dark can have their colour lightened by heating to 800 – 1400
Degree C in oxidizing conditions (with oxygen present). The very pale
brownish – grey material from srilanka that is called geudaq can be changed
to a blue by heating to temperatures of 1500 – 1900 degree C in reducing
conditions (without oxygen present). Variations in temperature and
conditions allow more subtle colour changes, some of which only reach just
beneath the surface, while others alter the whole stone. For over 100 years
brown topaz has been heated to give more attractive pink, and amethyst (the
purple variety of the quartz) has been altered to the less common citrine
(orange – brown variety).
As well as
heating, gemstones can be irradiated to improve or change their colour. They
may be exposed to gamma rays or bombarded by particles such as electronics,
neutrons, protons or alpha particles. Much colourless topaz is irradiated
and heat – treated to blue.
have flaws or cracks that detract from their beauty. The traditional method
of oiling emeralds is a simple process Essentially, it just involves
immersing a stone in oil or wiping the surface with an oily cloth. The oil
is then drawn into the cracks, with the result that they are less noticeable
and the stone appears to be clearer and of a better colour.
various colourless oils, waxes and plastics are used on a number of
different gemstones. Some remain liquid; others , such as resin, set hard
within the stone or a surface coating. Turquoise, lapis lazuli, jade and
some chalcedonies are dipped in liquid paraffin wax or given a surface
coating of wax, after polishing, which penetrates the stone to fill crack
and gives a better surface colour. In addition, coloured oils and resins are
also used. Matching the colour of the oils or resins to the stone improves
the colour as well as hiding the cracks.
Where a stone
has been oiled it may feel ‘only or may leave a stain when wiped with an
absorbent material such as a tissue. Years of wear or cleaning with
ultrasound may displace any oils and fillings, with the result that the
cracks in the stone will become more obvious and, in the worst case, the
stone will fracture.
and stains can also be used on some gems. Agate is dyed to imitate many gems
or to give bright, but a rather unnatural – looking pinks, greens and blues
for decorative carved pieces. Quartz rocks have been dyed green to imitate
jade and red to imitate ruby.
stones involves placing a piece of reflecting material, such as a metal
foil, behind the stone to change or improve the colour and make the stone
appear brighter. Foiling was used in Britain, particularly during the
Victorian era, to enhance costume jewellery made of paste (glass). Thin
films of gold, Silver and other metals can be deposited on the surface of
gemstones and crystals to give a surface ‘bloom;. When the back of the
stone is coated, the mirror – like qualities increase the reflectivity and
the stone appears brighter as well as taking on the colour of the coating.
Quartz crystals coated with a surface film of gold to give a pale blue
colour are called ‘aqua aura’.
gemstones are almost exact copies of natural gem minerals. Made under
laboraty conditions, most are manufactured by melting or dissolving the
appropriate mineral ingredients and colouring agents, then allowing the
molten mass or solution to crystallize at strictly controlled pressures and
temperatures. The resulting crystals are virtually identical in both
composition and crystal structure to the natural gem mineral, so posses
similar optical and physical properties.
gem – quality synthetics were the rubies produced in 1902 by the Frenchman
Auguste Verneuil, using a flame – fusion process.
spinels and sapphires followed soon after, and this method has proved so
cheap and fast that it is still used to produce most synthetic rubies,
sapphires and spinels. Emeralds, however, are made by other processes, and
may take nine months to crystallize from a melt. Because of this, synthetic
emeralds are more expensive, but may still be ten times cheaper than good
natural stones. Today, as technology develops, it is possible to synthesize
more and more gemstones, including opal, chrysoberly and diamond. Sythetic
gemstones can also be used imitate other gemstone; for instance, aquamr