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August 1999

Diamonds:Gemology

Just Your Type

A closer look at why the classification of diamonds by type may help distinguish which diamonds respond most effectively to enhancements like GE-POL

A heated discussion of diamond types revolves around Lazare Kaplan International's recent decision to market diamonds that have undergone a secret General Electric process to improve their color (Professional Jeweler, May 1999, p. 45, and July 1999, p. 23).

The most recent news is that the GE-POL processed diamonds could be Type IIa diamonds rather than Type Ib, as previously thought. Say what? Let's review diamond classifications to better understand what difference this makes.

Diamonds are not created equally – in nature or in a laboratory. One way to group them is by color. Diamonds can owe their color to trace elements, atoms or groupings of atoms that may cause color centers. What causes color – or the lack of it – guides much of the research into diamond treatments. These are the diamond types:

Type I
Type I diamonds make up about 99% of all diamonds. They contain an abundance of nitrogen atoms and are electrical non-conductors. They are further broken down as follows:

  • Type Ia: By far the most common in the Type I category, these diamonds contain clusters of nitrogen atoms as impurities in the crystal lattice. When nitrogen atoms cluster into pairs, they don't cause color because they absorb ultraviolet light beyond the visible blue end of the spectrum. These diamonds show a dark line at the 415 nanometer line in the spectroscope. This category comprises colorless and some yellowish diamonds.
  • Type Ib: These are much rarer than Type Ia diamonds (less than 1% of all Type I). They contain nitrogen atoms that are dispersed throughout the crystal lattice. When nitrogen atoms are dispersed, they can absorb light in the blue end of the spectrum, allowing yellowish to fancy yellow stones with stronger color than Type Ia. Type Ib and mixes of Type Ia/Ib can be subjected to high pressure and high temperatures (HPHT) to lose color.
Type II
A small number of diamonds are Type II.
  • Type IIa: These are rare diamonds with exceptionally pure chemical composition and contain no nitrogen or boron. They are often very large and usually colorless, though they may be pink, brown or blue-green. They are inert to shortwave ultraviolet radiation and don't conduct electricity but are efficient conductors of heat.
  • Type IIb: These are even rarer than Type IIa diamonds. Boron substitutes for some carbon atoms in Type IIb diamonds. They are electrical semiconductors and are extremely sensitive to temperature changes. They phosphoresce to shortwave ultraviolet. Most blue diamonds are Type IIb.
Diamond Types and Enhancements
When Lazare Kaplan announced its subsidiary, Pegasus Overseas Ltd., would sell diamonds subjected to the GE process (now to be laser-inscribed GE-POL), the industry speculated it involved HPHT techniques. That theory drew credence from a 1977 GE patent on diamond processing that says Type Ib diamonds can be subjected to temperatures of 2,200°C and pressure of at least 70 kilobars to improve color. This process reportedly converts at least 20% of Type Ib nitrogen to Ia (a lighter, more desirable color). But Lazare Kaplan says GE has not patented the process used to whiten the diamonds Pegasus Overseas Ltd. sells.

Critics of the HPHT theory, including Martin Haske of Adamas Gemological Laboratories, Brookline, MA, say also the HPHT process would be prohibitively expensive and impractical.

Heating Theories Heat Up
Different theories about heating are emerging. Christopher Smith, director of the Gübelin Laboratories in Lucerne, Switzerland, has advanced a new theory about the GE process. He believes it involves the healing of structural defects related to plastic deformation through controlled heating. "Plastic deformation is a color-causing mechanism in natural brown, pink and purple diamonds whose color may be influenced also by certain impurities," he says. "Plastic deformation occurs when pressure causes a shift in the crystal lattice along growth planes, creating defects in the crystal structure. The GE process doesn't appear to involve the clustering or diffusion of nitrogen impurities, as previous theories proposed."

Smith cautions the sample of the diamonds his lab received for study was small, mostly Type IIa diamonds, and may not be a total representation of the GE-POL diamonds. "The sample diamonds were type IIa, D or E color and at least VS1 clarity," he says. "If the process is as we suspect, then it should be disclosed as a treatment. While the quantity of these diamonds hitting the market is small, they're hitting an important segment of the market – the higher colors."

Separating Types
How do labs separate Type I from Type II diamonds? Craig Slavens, manager of the diamond department at GQI Laboratory in Los Angeles, CA, says there is at least one "down and dirty" way to identify the treatable Type I diamonds. "Type I diamonds are not transparent to ultraviolet light, but Type II diamonds are," he says.

He points to an easy test described in the book Fluorescenceby Manuel Robbins (Geoscience Press, 1994): Because Type I diamonds don't transmit any shortwave ultraviolet light, the author suggests building a daylight-tight environment and putting a diamond on a mask over a pinhole. Place a shortwave light over the diamond and position a gem that fluoresces strongly to shortwave ultraviolet – such as scheelite – under the opaque surface and pinhole. Any shortwave light transmitted through the diamond will cause the scheelite to fluoresce blue. If the scheelite fluoresces, you have a Type II diamond. The spring 1999 Gems & Gemology,the Gemological Institute of America's quarterly journal, describes a similar procedure.

– by Robert Weldon, G.G.



Copyright © 1999 by Bond Communications.



 

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