Professional Jeweler Archive: GE/POL Yellowish Green Diamonds Enter the Marketplace

May 2000

Diamonds/Gemology


GE/POL Yellowish Green Diamonds Enter the Marketplace

Color stems from high-pressure/high-temperature conditions


Researchers at General Electric recently introduced yellowish green diamonds processed using high-pressure/high-temperature conditions. Laboratory irradiation is not part of the coloration process. Exposure of selected diamonds to HPHT conditions creates an optical center that produces the striking yellowish green appearance. These diamonds will be marketed by Pegasus Overseas Ltd., a subsidiary of Lazare Kaplan International, New York City, and will be laser-inscribed on the girdle to provide the necessary disclosure. GE has related pending patent applications. This article briefly describes these new diamonds and their gemological properties.

Background

GIA first encountered what was believed to be a new kind of diamond color treatment in fall 1996 and published a brief report on it in the GIA Gem Trade Lab Notes section of the Summer 1997 issue of Gems & Gemology (Reinitz, I., and Moses, T.M. GIA Gem Trade Lab Notes: “Treated-Color Yellow Diamonds with Green Graining, Gems & Gemology, Vol. 33, No. 2, p. 136).

This initial report, which described the unusual gemological properties of these yellow green diamonds, speculated their coloration involved heating of the diamond at very high temperatures and possibly at high pressures. We were later informed these diamonds were from Russia. Van Bockstael described some diamonds with similar gemological characteristics and said they had undergone HPHT treatment in Russia (Van Bockstael, M. “Enhancing Low-Quality Coloured Diamonds,” Jewellery News Asia, No. 169, September 1998, pp. 320 and 322).

Henn and Milisenda reported on similar material from Russia (Henn, U., and Milisenda, C.C. “Gemmologische Kurzinformationen: Ein neuer Typ farbbehandelter Diamanten,” Gemmologie – Zeitschrift der Deutschen Gemmologischen Gesellschaft, 1999, Vol. 48, No. 1, pp. 43-45).

Buerki et al. presented a more technical report about the optical center associated with the strong green luminescence that is a characteristic of these diamonds. (Buerki, P.R.; Reinitz I.M.; Muhlmeister, S.; and Elen, S. “Observation of the H2 Defect in Gem-Quality Type Ia Diamonds,” Diamond and Related Materials, 1999, Vol. 8, pp. 1,061-1,066).

Recently, Novatek Co., Provo, UT, announced it uses an HPHT process to produce a similar color in diamonds (Templeman, T. “NovaDiamond Introduces New Enhancement,” Rapaport Diamond Report, Vol. 23, No. 1, pp. 1, 29-30).

GE developed HPHT technology to produce yellowish green in diamond in the mid-1990s. The decision was made not to commercialize the product at that time. Over the next three years, hundreds of experiments led to the development of an optimum color, as well as a color grade scale that GE and POL representatives could use to describe the product. The starting materials used for this process are Type Ia diamonds with a mostly brown or brown-yellow body color.

It should be noted that on rare occasions, yellowish green diamonds do occur naturally in localities such as Sierra Leone, the Ivory Coast and Brazil. For example, Collins reported on a natural diamond that showed similar yellowish green and spectrum as the new GE/POL diamonds (Collins, A.T. “Color Centers in Diamond,” Journal of Gemmology, 1982, Vol. 23, No. 1, pp. 72-74).

An interesting property of these processed diamonds is their color-change (chameleon-like) behavior upon heating. Natural-color chameleon diamonds are extremely rare and can be quite valuable. When these GE/POL diamonds are heated to 550°-650° Celsius, their color changes from a yellowish green to a pure “emerald” green. This color change is not permanent; on cooling of the diamond to room temperature, the green appearance persists for about 10 to 15 minutes before the diamond returns to the original yellowish green. Note this is opposite to the change observed in natural-color chameleon diamonds, where heating causes an initially grayish green diamond to change temporarily to yellow. (Fritsch, E.; Shigley, J.E.; Moses, T.; Rossman, G.R.; Zucker, B.; and Balfour, I. 1995. A Green Diamond. Edited by D.J. Content, W.S. Maney & Sons Ltd., Leeds, United Kingdom, p. 42.)

To understand these GE/POL diamonds better, GIA documented the gemological properties of 31 diamonds representative of the color range that can be produced by General Electric. These diamonds were investigated also using visible and infrared spectroscopy at GIA and General Electric.

Gemological Characteristics

Color. The colors of these 31 diamonds are in the greenish yellow to yellow green range and are moderately to highly saturated, according to the terminology used by the GIA Gem Trade Laboratory for natural-color diamonds. For convenience, the colors of these GE/POL diamonds are described simply as “yellowish green” in this article. Because of luminescence, the green component of the color is more pronounced when these diamonds are viewed in daylight or with a strong incandescent light source.

For research and marketing purposes, GE devised a color scale for these diamonds based on colorimetric measurements. The scale ranges from 10 to 1 and is used to describe the intensities of the body color and daylight fluorescence of GE’s yellowish green diamonds. A rating of 10 represents the most intensely colored yellowish green diamond with the strongest fluorescence and a 1 indicates the lowest saturation of color and degree of fluorescence.

Luminescence. When examined in a darkened room with an ultraviolet lamp, these GE/POL diamonds exhibit strong greenish yellow fluorescence to longwave ultraviolet radiation and a similar color with slightly weaker intensity to shortwave UV. On occasion, the fluorescence has a chalkiness or haziness that may be distributed unevenly. Despite the strong fluorescence, there is very weak to no phosphorescence. Such fluorescence occurs in natural and treated-color yellowish green diamonds (Collins, A.T. “Color Centers in Diamond,” Journal of Gemmology, 1982, Vol. 23, No. 1, pp. 37-75).

Clarity and Internal Features. The GE/POL diamonds exhibited a broad range of clarity grades and internal features (such as crystals, feathers and cleavages). Most of these diamonds displayed well-defined color zoning that was related to brown to yellow octahedral graining (best seen with a diffused light source in a gemological microscope). Illumination with a strong light source, such as a fiber-optic lamp, caused these colored graining planes to luminesce bright green.

Spectra. The spectra of these 31 GE/POL diamonds were recorded using visible and infrared spectroscopy equipment that has become standard in gemological laboratories that issue origin-of-color reports on colored diamonds. In general, the spectra of these GE/POL diamonds exhibited a strong 415 nanometer line (the N3 optical center), the 498 and 503nm lines (the “503 pair,” which are due to the H3 center), a line at 870nm (of unidentified cause) and a line at 985nm (the H2 center). When observed with the desk-model spectroscope, these diamonds also displayed a pair of emission lines at about 513nm and 518nm (as illustrated in the spectra on page 143 of Handbook of Gem Identification, 12th ed., by Richard T. Liddicoat Jr., 1987, Gemological Institute of America, Santa Monica, CA).

Conclusion

Though these new GE/POL yellowish green diamonds display a strong green luminescent body color and greenish yellow fluorescence, these features alone are not proof that a colored diamond has been subjected to an HPHT process. Conclusive identification will require consideration of other gemological properties as well as testing with spectroscopy equipment.

GIA believes that using a combination of gemological features and spectra, it is able to identify any GE/POL yellowish green diamonds that would be submitted to the laboratory. Colored diamonds identified as having been subjected to HPHT conditions are referred to as “treated color” on GIA Gem Trade Laboratory reports.

With the greater demand for colored diamonds, it should not be surprising to see these new kinds of HPHT-processed diamonds being offered for sale in the jewelry marketplace. Work done by General Electric and others with this technology has resulted in the introduction of yellowish green diamonds that duplicate a color that is rare among natural-color diamonds. Members of the diamond trade and those in gemological laboratories need to recognize that new technologies are being developed and to keep updated on the gemological characteristics of these processed diamonds.

GE/POL is evaluating consumer interest and likely demand for these unusually colored diamonds. GE/POL representatives are working with a select group of retailers to develop custom-designed jewelry specifically for these yellowish green diamonds that will maximize their color and appeal to the consumer. As with the colorless to near-colorless GE/POL diamonds, these yellowish green diamonds will have a laser inscription on the girdle facets to allow practical identification and full disclosure.

GE/POL and GIA plan to continue their close cooperation in researching new products as they are introduced. A more detailed report on the new yellowish green diamonds will be prepared for an upcoming issue of Gems & Gemology.

Acknowledgement

This article is based on research funded in part by a generous donation from Argyle Diamonds and ongoing support from De Beers.

– by Thomas Anthony, John Casey, and Suresh Vagarali of General Electric Co. and James Shigley, Thomas Moses, and Matthew S. Hall of GIA

Using high pressure and high temperature, GE has produced highly saturated yellowish green in diamonds. These GE/POL yellowish green diamonds set in multicolored gold range from 0.40 to about 1 carat.
This group of new GE POL diamonds, which range from 0.45-0.70 carat, is typical of the color range available in material being produced.
Two pear-shaped GE/POL greenish-yellow diamonds (1.23 and 1.33 carats).


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