Winter 2009

Letters

More on the “Fluorescence Cage”

We read with interest the article “‘Fluorescence cage’: Visual identification of HPHT-treated type I diamonds,” by Inga Dobrinets and Alexander Zaitsev in the Fall 2009 issue (pp. 186–190). The authors claim that the fluorescence cage they observed is proof of high-pressure, high-temperature (HPHT) treatment of type I diamonds.

The features described are identical to those published over eight years ago by Pierre Yves Boillat and coauthors (“Luminescences sous excitation visible des diamants noirs irradiés: les luminescences d’arêtes” [Luminescence of irradiated black diamonds under visible light excitation: Facet edge luminescence], Revue de Gemmologie afg, No. 141–142, 2001, pp. 37–41; see also figure 1). This article has an English abstract and figure captions, as it was felt the results could interest the international gemological community. “Facet-edge luminescence” is illustrated in 17 photographs.

Hence, fluorescence cage luminescence is not characteristic of HPHT-treated diamonds. It is common among certain classes of irradiated (and sometimes annealed) diamonds. A similar phenomenon can be observed as well on natural, untreated diamonds, although there are minor differences. Ascertaining unambiguously the cause of this curious phenomenon requires further experimentation.

Emmanuel Fritsch
University of Nantes and IMN, Nantes, France

Franck Notari and Candice Grobon-Caplan
Gemtechlab, Geneva, Switzerland

Thomas Hainschwang
Gemlab, Balzers, Lichtenstein

Reply: The reason we concluded that the “fluorescence cage” is related only to HPHT-treated type I diamonds was a simple one: We never saw this effect in diamonds except in HPHT-treated ones, and at the time of our G&G publication, we were unaware of the Boillat et al. article (for which we offer our apology). We did not observe the effect on irradiated diamonds that were occasionally checked together with HPHT-treated ones with the fluorescence microscope. Although one heavily electron-irradiated diamond of deep greenish blue color did show a weak “cage,” this observation was not anomalous because that diamond also revealed clear features of prior HPHT treatment. We interpreted this fact as evidence of the high stability of the “cage”—that is, that it can survive heavy irradiation. Thus, from a practical point of view, we have no doubt that the “fluorescence cage” is a very strong indicator of HPHT treatment.

Concerning the occurrence of the “cage” effect in natural (untreated) diamonds, of course, it is theoretically possible. Indeed, some natural-color diamonds may reveal characteristic features of HPHT annealing or irradiation that has occurred while the gem is in the earth. In our experience, however, such diamonds are extremely rare.

Concerning the physics of the effect, we are not certain that the “fluorescence cage” seen in HPHT-treated diamonds is fully identical to the “facet-edge luminescence” observed in the irradiated stones. In some HPHT-treated diamonds we examined, the “fluorescence cage” has an appearance very different from that of the regular “cage” and that of the “facet-edge luminescence.” These results will be published soon elsewhere.

Alexander Zaitsev
College of Staten Island, New York

Inga Dobrinets
European Gemological Laboratory, New York

More on Green Amber
I am delighted that a comprehensive paper has been published about the effects of the treatment performed on various ambers and copals to turn them green (A. Abduriyim et al., “Characterization of ‘green amber’ with infrared and nuclear magnetic resonance spectroscopy,” Fall G&G, pp. 158–177). The tests conducted by the authors were extensive and thorough, but it appears that only the outer surface of the resins altered by treatment was tested. None of the samples was cut in half to see what had happened to the interior.

Awhile back I had some pieces of treated Colombian copal tested in London, and the inside of the material gave a different FTIR reading from the outer surface, indicating that the treatment did not penetrate all the way through.

I have long maintained that, no matter what treatment it is subjected to, the immature resin copal should not be referred to as amber. The fact that this treated copal may not have hardened all the way through means that the center is still copal, even if the outer surface resembles some kind of amber in appearance and in testing.

I am worried the Laboratory Manual Harmonization Committee (LMHC) has now decided that a sample should be called amber when it is not possible to determine by testing exactly which resin has been used as a starting material—albeit with the addition of some words to say that it shows signs of treatment and may be derived from copal.

Some of the green material on the market today derives from amber, but much of it uses Colombian copal as the starting material. I feel that we degrade true amber by using the word amber for material that may well be copal.

In my experience, the general public is already afraid to buy amber because they have heard that much of it is treated or even faked. Surely, by accepting that we can call copal amber, we risk damaging this shaky reputation even further?

Maggie Campbell Pedersen
Editor, Organic Gems

Reply: I appreciate Ms. Campbell Pedersen’s comments, and welcome the opportunity to clarify some items. First, in this study, Fourier transform infrared spectroscopy (FTIR) and nuclear magnetic resonance (NMR) were used to study the bulk structure of several green amber samples, both on the surface and in the interior. The FTIR spectra of the surface and internal sections of “green amber” created from Brazilian and Colombian copal presented similar features. However, there is a possibility that some commercial treated “green amber” may show spectral features of copal due to insufficient treatment or an incorrect approach being applied in the heating process; this material should therefore be described as copal.

Second, there appears to be a misunderstanding about LMHC nomenclature for “green amber.” LHMC members will not call all green or even yellow-brown resins amber without an accurate analysis. If the presence of the Baltic shoulder and 820 cm⁻¹ absorption features in the FTIR spectrum of green and yellow-brown treated material is confirmed, the material should be called amber and the following comment placed on the report: “Indication of heating, this resin has been processed by heat and pressure.”

If the material lacks the Baltic shoulder but shows amber-like absorptions, and it is extremely difficult to identify the starting resin, the item should be called amber but only after a full analysis of the surface and/or interior (if permitted by the client). The statement “Indications of heating, this resin has been processed by heat and pressure and may have been derived from copal” would then be placed in the comments.

Further, the LMHC position on this material is still being finalized. If the interior of the sample gives an FTIR reading different from that of the outer surface—that is, the outer surface resembles some kind of amber and the center is still copal—it should be called copal only. The LMHC is presently considering the scenario where permission is not given to cut a sample of green amber in half, and will discuss whether a comment such as “Green amber is commonly derived from copal material” should be added to reports.

Ahmadjan Abduriyim
GAAJ-Zenhokyo Laboratory