A number of techniques are used to improve the color and appearance of natural and synthetic gemstones. The purpose is to increase their beauty, desirability, and saleability. Probably the oldest method is that of heat treating gemstones to improve or change its color. The heating of carnelian has been transported 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 which others maybe temporary. For example stains and fills may leak, some heated, and irradiated stones may fade or revert to their original color.
Most rubies and sapphires are heat treated to improve their color. Sapphires considered too dark can have their color lightened by heating to 800-1400 degrees Celsius in oxidizing conditions. The very pale brownish gray material from Sri Lanka called geuda can be changed to a blue by heating to temperatures of 1500-1900 degrees Celsius in reducing conditions (without oxygen present). Variations in temperature and conditions allow more subtle color changes, some of which only reach just benefit the surface while others alter the whole stones. For over 100 years brown topaz has been heated to give a more attractive pink and amethyst has been altered to the less common citrine.
As well as heating, gemstones can be irradiated to improve or change their color. They may be exposed to gamma rays or bombarded by particles such as electrons, neutrons, protons, or alpha particles. Much colorless topaz is irradiated and heat-treated to blue.
Most emeralds have flaws or cracks that detract from their beauty. The traditional methods of oiling emeralds are 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 better color.
Nowadays varied colorless 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 as 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 penetrated the stone to fill cracks and gives a better surface color. In addition, colored oils and resins are also used. Matching the color of the oils or resins to the stone improves the color as well as hiding the cracks.
Where a stone has been oiled it may feel oily 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 worse case the stone will fracture.
Colored dyes and stains can also be used on some gems. Agate is dyed to imitate many gems or to give bright but rather unnatural looking pinks greens and blues for decorated carved pieces. Quartz rocks have been dyed green to imitate jade and ref 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 color and make the stone appear brighter. Foiling was used in Britain specifically during the Victorian era to enhance costume jewelry made of paste (glass). Thin films of gold, sliver and other metals can be deposited on the surface of gemstones and crystals to give the surface a 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 color of the coating. Quartz crystal coated with a surface film of gold to give a pale
Source by Christine Breen