The oldest method of enhancing the color of gems was the use of foil backings.

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Altering Gemstone Color

The oldest method of enhancing the color of gems was the use of foil backings. Modern methods are much more subtle and difficult to detect. The color of opal may be improved by coating the back with a black substance. Occasionally diamonds are "painted" with a pale-colored dye to offset a yellowish tinge, but the coating soon wears off. Quartz is sometimes stained or dyed to resemble jade or tourmaline. Chalcedony is porous and readily absorbs dyes to produce a variety of bright-colored stones.

Black onyx is made by soaking grayish-colored chalcedony in a sugar solution and then blackening in sulfuric acid. This process leaves very tiny particles of carbon in the pore spaces of the chalcedony.

Thereare many ways to improve the color of turquoise, such as by soaking in wax (paraffin) or impregnating with plastics. Such methods are usually detectable, but should generally be suspected if turquoise of deep-blue color is presented at a modest price. Grayish jadeite can be stained to produce an "Imperial" color, or dyed an intense mauve.

DIAMONDS, GEMSTONES & CRYSTALS
FACTS & FIGURES

There is natural mauve jadeite, but the dyed material is much darker in hue. Serpentine, a material not related to jade, can also be stained a rich green color that resembles Imperial jade.

Heating and Irradiation
The effects of heat and irradiation on gems are sometimes unpredicta¬ble. In other cases they are used to advantage in improving gem color. Topaz, for example, occurs in various colors. Pale-blue topaz is not uncommon, but deep, intense blue stones do not occur in nature. Such gems can be produced by gamma-irradiation of certain colorless topaz. This treatment turns the material greenish-brown, but heating produces a rich blue color. Some golden or yellow topaz can be heat-treated to produce a pink or purplish-red color.

The color o( pale-brown or "sherry"-colored topaz can sometimes be improved by gamma irradia¬tion, but heating or exposure to sunlight usually reverses the process. Color fading of natural brown, sherry, and some blue topaz is not uncommon. Unfortunately, in the case of blue topaz there is no current way to detect color enhancement by gamma irradiation. ,Although some fading may occur, beyond a certain point the color seems to be stable.

Heating usually lightens the color of tourmaline, but sometimes a dark-green stone can be made an attractive emerald color. Gamma irradiation of tourmaline produces spectacular color changes. Pale-pink and some colorless stones may turn dark pink. Medium-pink material may turn yellow, and blue gems may turn purple. Pale-yellow tour¬-maline can be made a peach color by gamma rays.

Heating is standard procedure with zircon, turning the drab and 148 unappealing brown and green material into lovely and desirable color-less and blue gemstones.


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