Green Envy

March 1999

Gemstones & Pearls:Gemology

Green Envy

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.

Gemological Pointers
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:

   Emerald  Tourmaline  Topaz
 Refractive Index  1.577-1.583  1.624-1.644  1.619-1.627
 Birefringence  0.006  0.20  0.008

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.


 

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