Gemstones & Pearls:Gemology
Diffusion treatment turns this topaz bright green
A steady army of bright green topaz is marching into the market. The
round, brilliant and emerald cuts in calibrated sizes are from Leslie &
Co., Irvine, CA.
Because bright green topaz doesn't exist in nature and could be confused
with other green gems, you may wonder how it's done and how to distinguish
it from other green gems.
The product starts as a colorless, "eye-clean" topaz from Brazil.
A color-imparting chemical is diffused into the surface under high temperature
(until now, only corundum has been diffusion-treated successfully).
While topaz has been coated and irradiated to create green before, diffusion
treatment results in a depth of color not seen previously. Producers tout
the permanence of the color and the durability of the gem after treatment.
"An added benefit is that the stones acquire a greater degree of scintillation,"
says Doug Jeffery, director of sales for Leslie & Co.
The company sells the green topaz with disclosure cards indicating the
origin and enhancement process clearly and providing information for bench
repairs and cleaning.
The diffusion process creates a color that is only "skin-deep,"
so you and your customers must be careful don't scratch the surface
or repolish normally. Some repolishing is possible, says Leslie & Co.,
depending where the scratch occurred. Pavilion scratches are harder to fix
without losing color.
Separation from emerald and tourmaline is easy because of differences
in refractive index and birefringence:
| Refractive Index
Emeralds also are generally more included than tourmaline and topaz.
Professional Jewelernoticed microscopic chemical stains in the
green topaz examined at 20X magnification that appear as color concentrations
in surface pits and uneven coloration from one facet to another. These features
are typical of a diffusion process and are not seen on emerald or tourmaline.
The green topaz sells for $50 to $70 per carat keystone. "We've
received feedback that this is high for topaz," says Jeffery. "But
we point out the products are eye-clean, require a minimum 80% brilliance
and are subjected to rigorous quality control standards that weed out 50%
of the material. After we explain that, we get agreement the price is fair."
Leslie & Co.'s sales office is in San Francisco, CA, (415) 775-3997.
by Robert Weldon, G.G.
Copyright © 1999 by Bond Communications.