May 03, 2000 - Review Board members Prof. Dr. E. Fritsch and Prof. Dr. H. A. Hänni recently co-authored an article with J.-P. Chalain titled "Identification of GE POL diamonds: A second step" (Journal of Gemmology, Vol. 27, No. 2, pp. 73-78). The authors studied 10 untreated and five GE POL diamonds (all type IIa), and tentatively propose that GE POL diamonds can be identified using a two-step process: The first step is to determine whether the sample is type IIa; this was done by the authors using the new SSEF Type IIa Diamond Spotter, which is based on the transparency of these diamonds to short-wave ultraviolet radiation. The second step is to look for subtle luminescence features related to N-V centers in the type IIa diamonds with a Raman spectrometer. Using 514 nm laser excitation, all five of the GE POL diamonds studied showed a luminescence peak at 3760 cm^-1 (corresponding to 637 nm), whereas none of the seven untreated D-color diamonds showed this feature. The authors also noted the 637 nm luminescence in all three of the untreated brown diamonds studied, and suggest that the presence of N-V centers in near-colorless diamonds is probably indicative of GE POL treatment.

Review Board member A. T. Collins and co-authors H. Kanda and H. Kitawaki have recently published an article titled "Colour changes produced in natural brown diamonds by high-pressure, high-temperature treatment" (Diamond and Related Materials, Vol. 9, No. 2, pp. 113-122). The authors used optical absorption and luminescence spectroscopy to investigate three treated-color yellow-green to green diamonds from NovaDiamond Inc., as well as 14 brown diamonds that were characterized before and after high-pressure high-temperature (HPHT) treatment. If the starting material has a very low concentration of nitrogen (such as that used for the type IIa GE-POL diamonds), HPHT treatment causes some healing of the dislocations responsible for the brown color, leading to a diamond with less color. The samples processed by NovaDiamond are mainly type Ia diamonds, which typically contain a few hundred ppm nitrogen. In most natural diamonds the nitrogen is present as pairs, or as groups of three or four nitrogen atoms surrounding a vacancy in the diamond crystal structure. HPHT treatment of brown type Ia diamonds produces two effects: (1) When the dislocations that were initially present begin to heal, additional vacancies (V) are released and are trapped by the nitrogen pairs (N-N) to form N-V-N centers. (2) Some of the nitrogen pairs dissociate to produce single nitrogen atoms; this allows a few N-V centers to form, and also influences the charge state of the N-V-N centers. Uncharged N-V-N centers produce an absorption in the blue part of the visible spectrum, which gives the diamond a yellow color. Negatively charged N-V-N centers produce an absorption at the red end of the spectrum; when combined with the absorption in the blue region, this absorption results in a green or yellow-green diamond. If the nitrogen concentration is not too high, the absorption of light by the uncharged N-V-N centers also produces strong green luminescence, which greatly contributes to the overall color of the diamonds when they are viewed in daylight or with a strong light source. When colorless type Ia diamond containing nitrogen pairs (rather than larger nitrogen aggregates) is the starting material, the dissociation of the nitrogen pairs produces some single nitrogen atoms, resulting in the absorption characteristic of "canary" yellow diamonds.