Gem Enhancements

Gem Enhancements

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Many gemstones are altered to enhance their appearance. This may be done to produce colors not usually found in nature, to improve color, to improve clarity, to reduce porosity and stabilize color (by preventing absorption of discoloring oils and other substances), or to enhance durability. Depending on the stone and the treatment, such alteration may be easy or impossible to detect. It is unethical and unlawful to sell any artificially enhanced gems without full disclosure of information about the treatment. Unfortunately, full disclosure seems to be the exception rather than the rule in the marketplace. Here are a few examples of some of the methods frequently used: heat treatment, irradiation, impregnation,assembled stones.


Heat treatment

Many gems are routinely heated under controlled conditions to improve color (aquamarine, sapphire, ruby, tourmaline), alter color (sapphire, amethyst to citrine, topaz, zircon), or improve clarity (sapphire, ruby). Since natural heating also occurs (e.g., in volcanic areas), the artificial effects are sometimes indistinguishable from natural effects. In most cases, the results of heat treatment are permanent.

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Irradiation

Colorless topaz is irradiated in large quantities and then heat treated to produce various shades of blue. Yellowish diamonds are often irradiated to produce a wide variety of colors. Other stones, such as tourmaline, are sometimes irradiated to enhance or produce new colors. In many cases, the effects of irradiation are somewhat unstable and can be reversed by heating.

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Impregnation and chemical treatment

Turquoise is often very porous and is sealed with wax or plastic resin to "stabilize" and improve the color. Such material is very abundant and often not disclosed. "Black onyx" is almost always agate that has been impregnated with sugar, which is then carbonized by acid. Yellowish diamonds are sometimes coated on the girdle or pavilion with a thin bluish film to improve color. Jadeite is sometimes chemically "bleached" and impregnated to improve color, and this treatment can be difficult to detect.

In recent years, a new treatment for corundum has appeared, in which poorly colored corundum is heated in chemicals to deposit a thin (less than 0.5 mm) layer of enhanced color on the surface of the stone. These stones can be quite impressive, but recutting removes the surface coloration and results in a very disappointing stone. Such treatment is fairly easily detected by immersing the stone in a liquid with a high refractive index; the color appears to concentrate along facet edges. Until recently, only diffusion-treated blue sapphires were known, but the Fall 1995 issue of GIA's Gems and Gemology describes some diffusion-treated rubies, so other colors, and perhaps other stones, are likely to be entering the marketplace.

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Assembled stones

Thin seams of opal are often assembled with backing of opal or black onyx to produce a doublet, and a clear quartz top is added to produce a triplet. In addition to making otherwise unusable material useful, the dark backing enhances the play of color, and the quartz top adds to durability. Opal doublets and triplets must still be protected against heat and liquids. If the adhesive layer begins to break down, the stone's appearance is marred, and it is difficult, if not impossible, to repair the damage.

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