A Fancy Spectrum

Diamonds:Gemology

A Fancy Spectrum

Colored diamonds may be a big part of your future if their availability and growing popularity continue. Learn the basics so you can tell your customer their fascinating story

By curious flukes of nature, some very rare diamonds are born the colors of a rainbow. These are the diamonds for customers who have everything else, those who covet a jewel few others can own.

Once owned only by royalty, "fancy" diamonds have gone democratic this century, thanks to increased production in Australia, Brazil, Venezuela and parts of Africa where conditions are right for natural color.

 

Rough and cut diamonds in some of the rarest colors.

Their popularity and beauty also have spawned color-induced diamonds and synthetic colored diamonds. These are less expensive, of course, and further expand consumers' horizons in the diamond world.

Fancy This
A "fancy" diamond is a natural diamond of color – such as red, green, purple, violet, orange, blue and pink – and should not be confused with a "fancy cut," which refers to shape. Fancy colors vary from faint to intense.

Any diamond that falls between K and Z on the Gemological Institute of America's color grading scale can contain small amounts of yellow but not be considered fancy. Any yellow diamond beyond Z is a fancy color and, as such, commands a premium.

Causes of color – yellow and otherwise – are not as cut and dried as you might think. Infinitesimal impurities, irradiation or anomalies in crystal growth – or combinations of these factors – are the known causes of color. Here's a closer look at each one.

Prevalent fancy diamond colors include yellow and brown. Even some pinks are tinged with brown.

Impurities
It's curious how such practically negligible elements in diamond can cause such a riot of colors. For example, nitrogen (sometimes combined with hydrogen) results in many of the yellow and some of the brown shades.

Yellow diamonds are called "canaries" (referring to the yellow bird) or "cape series" (they were first found in Cape Province in South Africa). They are type Ia or Ib diamonds, which means they contain clusters or aggregates of nitrogen atoms. If your store has a spectroscope, you can see that Ia types show dark lines at the 415 nanometer mark, according to the GIA Diamond Dictionary. Ia types are quite prevalent in natural stones of weak, non-fancy yellow color.

Two extremely rare red and green rough diamonds. There's a reluctance to cut green rough because the color is often only "skin deep."

Natural Ib diamonds, which are related to saturated fancy yellows, are quite rare. Most synthetic fancy yellow diamonds are type Ib also. The color of natural light yellow diamonds can be deepened with irradiation and annealing (heating). Emmanuel Fritsch of Nantes, France, an expert on origin of color in diamonds, says combinations of irradiation and heating "cause color centers which reinforce the originally weak yellow color and, as a result, the treated gem becomes more marketable."

Type IIa diamonds contain boron or bits of nitrogen. While they are more often colorless, they also can be pink, blue-green or brown.

Type IIb diamonds are very rare and contain boron, which may substitute for some of the carbon atoms. They are most often blue.

Crystal Deformity
Some of the rarest and most coveted diamond colors are pink and red. The 20th century will be remembered for marketable quantities of these colors, mostly from the Argyle mine in Australia. Argyle has auctioned 50-60 of these diamonds annually for a little over a decade.

The cause of the color remains a mystery. Some speculate manganese inclusions impart color in thin bands in the crystal lattice. Others theorize it may be a perceived color due to crystal deformation – a missing atom causing a structural defect. It may be a combination of factors.

In an added twist, irradiation of type Ib synthetic diamonds has been known to create red.

Irradiation
Most green diamonds owe their color to natural irradiation, probably from radioactive minerals (such as uranium, thorium and potassium) in the kimberlite ore in which they are found.

Irradiation also causes some diamonds to turn brown or blue to violet.

Natural green is very rare. One way to tell the difference is to examine the "skin" of a cut green diamond. Color caused by irradiation is mostly concentrated in stains on the outer layer. Polishing can remove this layer and the color with it. Determination of color origin in green diamonds should be handled by well-qualified gemological laboratory.

  Colored Diamond References

  1. The Nature of Diamonds, edited by George Harlow. Cambridge University Press in association with the American Museum of Natural History. Cambridge, UK, 1998.
  2. Diamonds by Fred Ward, Gem Book Publishers. Bethesda, MD. Revised edition 1998.
  3. Fancy-Colored Diamonds by Harvey Harris, published in Lichtenstein by Fancoldi Registered Trust, 1994.
  4. Collecting and Classifying Coloured Diamonds by Stephen C. Hofer, Ashland Press, New York City, NY, 1998.
  5. Gems & Gemology, Winter 1994, "Color Grading of Colored Diamonds in the GIA Gem Trade Laboratory." John King, Tom Moses, James Shigley, Yan Lu.

– by Robert Weldon, G.G.



Copyright © 1999 by Bond Communications.


 

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