On the Surface

August 1998

Gemstones & Pearls:Gemology

On the Surface

Coating gems with precious metals can create unusual effects; coating them with other substances can create deceptions. Here's what you need to know

Gemstone coatings are out of the closet and going mainstream. One of the most interesting techniques lends a metallic look to colored gemstones.

One example is Tavalite gems, which generally are topazes coated with a microscopic layer of metallic oxide compound. The unearthly optical effect mixes a gem's transparency with a metallic luster. The resulting spectral hues are actually interference colors, resulting from light reflecting from the surface of the gem andthe inner surface of the coating. (The technology is not new; camera lenses are similarly coated to protect them and help prevent glare.) Tavalite gems are marketed by Deposition Sciences Inc., Santa Rosa, CA.

A commercial experiment with another gem in this coating category – black jade coated with thin layers of crystal gold – has been a huge success for Bill Heher Enterprises, Trumbull, CT. The company sold its entire supply at the Tucson gems shows before they ended this year.

Gold can be fused also to rough quartz and drusy quartz. Maxam Magnata, owner of the Tucson, AZ, company that bears his name, calls it a merger of metals and gemstones. Set in jewelry, he says, gold-coated gems "create a natural extension of your gold work."

A thin layer of metal over a gemstone can be vulnerable to wear and tear. For this reason, Magnata adds a thin layer of iridium (from the platinum group of metals) over the gold.

In addition, it's best to mount such gems in jewelry that's less prone to knocks and scrapes. And don't clean these gems in ultrasonic cleaners.

More Worrisome Coatings
A technology known as thin-film diamond coating is used already in computer technology. And the Gemological Institute of America reported on fragile gems such as opal and emerald receiving such coating on an experimental basis a decade ago.

This and some other coatings can raise red flags. While metallic coatings are obvious and done for certain effects, some coatings are done with the intention of deceiving buyers – making them think a gem is something it really isn't.

Gems have been coated with a variety of substances since at least the mid-1500s, as described in Benvenuto Cellini's Treatises on Goldsmithing and Sculpture.The usual reason is to change a gem's color. Such coatings include varnish, dye, paint and, more recently, plastic. For example, purplish coating on the back or hidden surfaces of diamonds, such as under the prongs, can notch up the color grade of certain yellowish stones. In other cases, such as with porous low-grade turquoise, coatings are used to "stabilize" the stone.

Careful jewelers shouldn't worry. Most coatings can be detected under magnification. Look for darker color concentrations in spots, peeling coatings, discoloration and gas bubbles in plastic.

– by Robert Weldon, G.G.

Cubic zirconia coated with the Tavalite process results in a medley of iridescent colors. Gem courtesy of Deposition Sciences Inc., Santa Rosa, CA. Drusy quartz is selectively or entirely coated with 23k gold. Gems courtesy of Maxam Magnata, Tucson, AZ.

Copyright © 1998 by Bond Communications.


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