All you need to know about gemstone treatments
The term "treatment" is defined as being any enhancement process (other than cutting and polishing) that improves the colour, clarity, feature, durability or availability of a gemstone.
The practice of treating and enhancing gemstones has existed for hundreds of years; there are some gemstones that would not even exist, or would be far more expensive than they now are, were it not for treatments.
The most common treatment of gemstones is heating; it causes the colour of a gemstone to lighten, darken or change completely, while also usually bringing about an improvement in clarity and brightness.
Heating is done at temperatures ranging from 450º to 1850º Celsius for a period of 2 to 12 hours. Heating is detectable only by trained observers in a laboratory and is usually irreversible. Naturally occurring Citrine is quite rare and so the abundance of Citrine (yellow, gold and orange) is the result of heat treating Amethyst. Heat treatment is required to produce Tanzanite in shades of violet and blue, and Pink Topaz would not be available without heat treatment. These treatments are the industry standard and keep gemstones affordable and available.
The following gems are routinely heat treated:
Paraiba Tourmaline (Neon Blue/Green)
Zircon (blue and colourless)
Note: Natural gemstone heating also occurs when gemstones are heated in volcanic areas.
Oiling/Filling is the intentional filling of surface fissures with a colourless oil, wax or resin to improve the gemstones appearance. When using an oil or fill it must have the same refractive index as the stone so that the light will pass into the gemstone and back to the eye eliminating all visible inclusions.
- Oiling of Emeralds is standard as they tend to have numerous natural inclusions and surface breaking fissures, though not every Emerald is oiled (fine untouched specimens will command astronomical prices). When the rough Emerald is mined it is thrown into a barrel of oil. When cutting, oil is also used as a lubricant; the colourless oil seeps into the fissures so that they are less visible to the eye. Only Emeralds with no surface fractures are not oiled, as no oil can penetrate the gemstone
Note: Emeralds or oiled gemstones should not be put into ultrasonic or steam cleaners as the oil may leach out the gemstone may then shatter. Undamaged gemstones can always be re-oiled.
Filling is used on gemstones with surface fractures or cavities. Glass, wax, plastic/resin or other materials are used to fill these holes. This is sometimes done to Rubies. With close examination by magnification you may be able to spot differences in surface lustre.
Irradiation is the bombarding of gemstone material with subatomic particles or radiation, releasing electrons from their normal location and moving them to a better colour-producing location. Sometimes irradiation is followed by heating to produce a better or completely different colour (blue topaz is the most common example).
Blue topaz, which occurs naturally, is pale in colour, but with irradiation, vibrant shades not found in nature have been created. Tourmaline can be irradiated to darken pink into red which is then indistinguishable from natural red gemstones.
Off-coloured diamonds can be irradiated and heated to be turned into intense greens, yellows, blues, browns and pinks. These gemstones are fairly common, though irradiated diamonds will sell for much less per carat than naturally coloured ones.
Pearls can be irradiated to produce grey or blue colours, although using dye to produce these colours is much more common. Irradiated Pearls will sell for about the same price as dyed Pearls.
The only gemstones that can be dyed are those with a molecular structure that will accept the dye and not release it. For example:
- Agate has fine capillary tubes allowing dye to be drawn into the centre of the gemstone.
Turquoise and Howlite are soft porous gemstones allowing dye to be absorbed, although some of these treatments may only be a few millimetres deep.
Without dyeing, there would be no Black Onyx (dyed Chalcedony). Dyeing of Chalcedony is prevalent and permanent.
To make some of these dyed gemstones colourfast they may later be impregnated with epoxy (stabilized).
Pearls if dyed correctly will also be colourfast (beware of cheaper dyed Pearls). This is the industry standard, as these colours do not occur in nature and so no deception is involved.
Impregnation and Stabilization
Impregnation is the infusion of wax or paraffin into a porous material. Stabilization is the introduction of a bonding agent, usually plastic, into a porous material. Of the two processes, stabilization is the most permanent. Impregnated pieces must be kept away from heat or the wax could melt or leak. Some gemstones are waxed on the surface to enhance lustre but this is not very common.
Impregnation and stabilization are common for Turquoise. The upside for stabilized Turquoise is that it will not absorb oils and discolour as untreated Turquoise can.
Bleaching is a process for organic gem materials such as Ivory, Coral, shell (such as Mother of Pearl) and for Pearls. It lightens the colour and is permanent and undetectable. No price difference will exist as a result.
Gemstones are crushed, dyed and then reformed using resin as a stabilizing agent. (For example; some Turquoise and Hematite)
Coating is a process that has been used for over 200 years, where a lacquer or film is applied to the surface of a gemstone, to improve its appearance. Today, coatings are increasingly used to change and improve the colour of gems. Mystic Topaz is an example of a coated gem.
A composite gemstone is a precious gemstone glued on to a base. For example, an Opal doublet has a black coating or Black Onyx base glued to the back to intensify the colour, or simulate a Black Opal. If a gemstone’s price is too good to be true, or if the gemstone is too perfect for the price, ask for a gemmologist's report.
Diffusion is the process where chemicals (used along with high temperatures) change the colour and/or create an asterism (star effect, for example; Star Sapphire) inclusion. Usually only the surface colour of the gemstone changes and only to a depth of about half a millimetre, so if the gemstone is chipped or recut the original colour is visible. The hardness and original physical properties of the gemstone are not affected by the diffusion process.
This process drills very small holes, via a laser, into a gemstone to provide access to an inclusion which detracts from the beauty of the gemstone. The inclusion can then be vaporized or bleached to make it less obvious (if it has not already been burned out by the lasering). Under magnification, laser holes are visible when viewed at the correct angle.
Lasering is sometimes used on Diamonds. A lasered Diamond would be classified in the imperfect or slightly imperfect category, regardless of the improvement in apparent clarity and should be priced accordingly.
Gemstones that are not enhanced
There are some gemstones that are not usually enhanced. These include Garnets and Spinel.
Technology in gemstone treatment is always changing and improving and many treatments are very difficult to detect.
With some gemstones enhanced by specifically defined treatments, a vendor should always disclose any known treatments or enhancements (remember, they may not always know themselves). Most vendors are honest and forthcoming, but it is YOUR RESPONSIBILITY TO ASK.
There is nothing intrinsically wrong with gemstone treatments as long as you are made aware of their use. New treatments are being developed all the time, and gemological testing centres are constantly revising and updating their testing regimes to combat the unidentified treatments that can slip through the cracks. Most gemstones (with the notable exception of Garnet) have a particular treatment, or series of treatments, that are commonly used to increase the marketability of the gemstone.