REAL AND FAKE
REAL AND FAKE
Any 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.
Imitation 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.
The most 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 thermal probe.
Introduced in 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.
However, this 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 3.22.
Treated 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 original colour.
Most rubies 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.
Most emeralds 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.
Nowadays 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.
Coloured dyes 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.
Foiling of 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’.
Synthetics 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.
The earliest gem – quality synthetics were the rubies produced in 1902 by the Frenchman Auguste Verneuil, using a flame – fusion process.
Synthetics 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.