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Natural Yellow Diamonds
Natural and synthetic yellow diamonds owe their color mainly to nitrogen; color centers due either to free single nitrogen as in pure Type Ib diamonds, or triple aggregates of nitrogen (N3 centers). Even pairings of nitrogen, like A centers (two nitrogens) and B centers (four nitrogens) do not cause color unless the A or B centers are bound with a vacancy (lattice defect) in the diamond. [A fairly complete summary of the causes of color in diamonds is available in US Patent # 4950463, (available on the IBM patent server at http://www.patents.ibm.com)]
Pure "cape series" or Type Ia diamonds are characterized by series of absorption lines with the major feature at about 415nm (N3 center) and 478nm (N2 center). Natural Type Ib diamonds typically contain no visible absorption lines. Figure 1 illustrates normal room temperature transmittance spectra (0% to 100.00%) of these two major diamond types. [We have cheated however, by showing a synthetic Type Ib diamond grown without the use of a nickel catalyst (the use of which would cause distinguishing characteristics which we will discuss later on )]. Both of the diamonds whose room temperature spectra are shown in Figure 1 have color which would be graded in the Y?Z to light fancy yellow range. The spectrum shown below represent what would typically be seen in a desk model diffraction grating spectroscope. In a "cape" series diamond, one typically can resolve a narrow band at 478nm, one about 450nm and a cutoff at 415nm, depending on the color intensity of the diamond. In the case of the natural (or Cobalt / Iron grown synthetic) Type IB diamond, there would be no visible absorption bands as with the Type Ia diamonds.
The separation of pure cobalt grown Type Ib synthetics from their natural Type Ib counterparts requires careful examination for metallic, sometimes magnetic inclusions and a structured fluorescence. Type Ib yellow synthetic diamonds grown with a nickel catalyst, as is seen as typical today for material imported from the former Russian republics, are easier to distinguish than synthetics grown with the use of a catalyst other than cobalt. It might be noted that the Type Ib spectra shown in Figure 1 had an abnormally pure yellow hue, unlike "most" of the typical yellow synthetics seen to date, which, in general tend to have a brownish component to their color. This is about to change.
Treated Yellows: Irradiated and Annealed
The typical treated yellow diamond apparently owes its color to the combination of nitrogen and lattice defects induced by ionizing radiation followed by an annealing process. The room temperature spectra of typical treated yellow diamonds are shown in Figure 2. [The vertical red lines denote the positions where the SAS2000
Spectrophotometer Analysis System has detected radiation induced damage, the green trace centered at 50% transmittance is the smoothed first differential of the primary transmittance spectra shown. This differential is of significant use in detecting the H3 and H4 centers which, in the case shown, occur in a region of the spectra where the transmission is rapidly rising, which hides the narrow absorption lines.] The main diagnostic characteristics indicating treatment are absorption lines at 594.5 and at the GR1 pair (741.1 & 744.6nm). Unfortunately, the GR1 pair exists above the typically visible spectral region and is also easily destroyed in Type Ia diamonds by annealing at 900 degrees Kelvin. The 594.5nm feature is also reduced by annealing, and can be somewhat difficult to see with visible spectroscopy at room temperature.
One of the clues to treatment lies in a slightly greenish tinge to the overall yellow hue of the diamond. It suggests that vacancies created by the radiation have coupled with A (nitrogen pairs) and B(four nitrogens) centers to produce the H3(503.4nm) and H4(497.0nm) optical centers which tend to give the greenish hue modifier, and well as strong greenish yellow fluorescence. These centers are also found in natural yellow and brownish yellow diamonds so they are not completely diagnostic of irradiation. They also have been found in High Pressure High Temperature treated diamonds.
Treated Yellows: High Pressure High Temperature (HPHT)
Although the technique to change color in diamonds by use of HPHT has been generally known since the 1970's, it has recently been used by NovaDiamond and General Electric to produce what mainly are highly fluorescent greenish yellow diamonds, starting from brownish rough. The author personally took part in treating a 2.03ct brownie at NovaDiamond in Provo Utah. The room temperature pre and post treatment spectra for this diamond are shown in Figure 3. The pre HPHT spectra shows a weak 415 line only. In the post HPHT spectra, conventional room temperature spectroscopy may reveal absorption features at the H3 and H4 centers. SAS2000 room temperature spectroscopy reveals the H2 center at 986.1nm, generally associated prior to the unveiling of the NovaDiamond product, as a radiation induced defect.
When SAS2000 Liquid Nitrogen Immersion Spectroscopy (LNIS) techniques are used, the spectra reveals a complex series of HPHT induced defects including 871nm. In the journal Diamond and Related Materials [Vol 8 (1999) pp 1061?1066], Buerki et al (from the GIA ) published an article "Observation of the H2 defect in gem?quality type Ia diamond." (GIA had received over 50 H2 type "green transmitter" diamonds at the GIAGTL in late 1996, which was highly unusual.) Their results in this article surmised that the H2 defect was believed to involve irradiation with a large dose of high energy electrons (3 to 4 Mev) followed by a high temperature annealing in air at temperatures around 1400C." This theory has of course been proven wrong, as the process involves only HPHT and takes only about two minutes to accomplish, as reported by EGLNY and also witnessed by the author.
The NovaDiamond HPHT "green transmitter" diamonds examined to date by the author generally exhibit strong long and short wave greenish yellow fluorescence. But this will probably change, as evidenced by HPHT diamonds released by General Electric and marketed by Lazare Kaplan under the Bellataire tradename.
Evidently, General Electric has been able to significantly reduce the greenish hue modifiers by apparently processing at a different temperature ? pressure point than NovaDiamond. The author examined four GE HPHT processed diamonds, and now owns one which has bluish white versus the typical very strong greenish yellow fluorescence of most HPHT yellow diamonds. The SAS2000 LNIS spectra of this diamond is shown in Figure 5, showing increased transmittance in the 400 to 500nm range. Whether or not this becomes the norm in HPHT diamonds remains to be seen, as this appears to be an exception, but indicative of a goal because of the better overall color.
Russian Synthetic Ni Catalyst Yellows
For years it seems, at least to the author, Russian yellow synthetic diamonds, in general, have been characterized by a brownish hue modifier, which made them marginally acceptable for use in jewelry, even though they were relatively inclusion free compared to near colorless synthetics. Most, if not all appear to have been grown using a nickel?iron process, in contrast to those samples seen by the author which had been grown by DeBeers. Figure 6 shows the LNIS spectra of one of these diamonds with the apriori location of nickel related absorption lines given by dotted lines. The strongest lines shown are at 732.13 and 710.72mn. Lawson and Kanda (reference Journal of Applied Physics 73(8) 1193) indicate that these lines would be completely annealed out by 1800C. The majority of nickel related lines can be annealed out at lower temperatures. Other nickel related lines (not a complete list) may be found at 658.3, 650.3, 647.6, 637.9, 553.1, 527.0, 519.7 517.5, 515.2, 510.1, 485.5, 472.3 and 467.0nm, but most are easily annealed out at temperatures at or below 1800C.
Russian Synthetic HPHT Nickel Catalyst Yellows
Recently, the author has seen an increasing number of nickel catalyst synthetics, which have a significantly reduced, if non existent, brownish hue modifier, in effect what now appear to be salable yellows. These diamonds have apparently been grown under a higher temperature? pressure environment than previous diamonds, or have been HPHT cycled at the end of their growth. They exhibit a characteristic strong slightly greenish yellow fluorescence, which is in contrast to the mostly inert brownish yellow synthetic diamonds previously examined by the author.
The room temperature spectra of this diamond, shown in Figure 7, exhibited characteristic nickel related absorption centers at 792.0, 732.13nm 710.72, 690.91, 671.09, 553.35, 527.00, 520.14, 519.70nm and at 517.50nm . Long wave and short wave fluorescence was strong yellow, phenomena associated with HPHT treated diamonds, but not seen previously in nickel catalyst yellows. This particular diamond also exhibited a "frosted" surface reaching feather with graphitization at the surface boundary, which is a characteristic associated with GEPOL treated colorless diamond.
Noticeable in this and other similar samples of current Russian synthetic yellows is the increased strength of the 731 and 710.72 nickel related absorption centers which apparently is related to a change in the growth process or post growth processing.
Conclusion
The recent four or five years has seen a significant explosion of new and/or improved techniques to alter or create yellow color centers in natural and synthetic diamonds. Unfortunately, a lot of the diagnostic characteristics for these treatments are beyond the range of conventional visual spectroscopy. The gemological appraiser will continue to be stumped by these diamonds unless he or she keeps up with the new technology being employed in both the processing and the detection of these evolving products. Invest in the detection technology, or relegate the detection to the relatively few labs currently equipped to render professional opinions on the origin and origin of color on yellow and other fancy colored diamonds.
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