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
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"
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.