Abstract
While the first part of this study took a detailed look at the properties, defects and classification of brown diamonds with deformation-related (DR) brown color and compared them to pink to purple to red diamonds, this second part covers diamonds with non-deformation-related (referred to as NDR in this study) brown color, including diamonds with treatment-induced brown color and synthetic brown diamonds. It was found that the natural NDR brown diamonds include CO2 and Pseudo CO2 diamonds as well as certain hydrogen-rich diamonds. Based on these, the new classification of NDR brown diamonds has been elaborated, resulting in 5 different classes. The detailed defect study performed has shown and confirmed the complexity of the CO2 and Pseudo CO2 diamonds; the probable link between structurally bound oxygen and some of the spectroscopic features such as the 480 nm absorption band is apparent in these diamonds. One of the most interesting findings was made through the low temperature NIR spectroscopy of some usually hydrogen-rich diamonds, which has defined a defect of great interest, the 1330 nm center; we suggest that this defect, together with the many lines in the 970 to 1000 nm range—referred to as the 990 nm series in this study—are responsible for the complex UV-Vis-NIR spectra seen of these diamonds. The results indicate that both features are nickel-nitrogen-related defects, the 1330 nm defect without involvement of hydrogen and the 990 nm series likely with hydrogen involved. Another surprising result was that during various treatment experiments performed we created dark orangish brown color in originally pale yellow “cape” diamonds by HPHT treatment at 2500 °C. It is suggested that the creation of this brown hue is related to the destruction or transformation of the N3 center at such extreme conditions.
Highlights
Brown diamonds that owe their color to defects that are not related to post-growth plastic deformation (DR = deformation-related [1], NDR = non-deformation related) are quite rare, in larger sizes (Figure 1); earlier studies of brown diamonds have shown that parcels of melee-sized brown diamonds contained approximately 1.3% of brown diamonds with no signs of significant plastic deformation, designated as CO2 diamonds and Pseudo-CO2 diamonds [2]
One of the most interesting findings was made through the low temperature NIR spectroscopy of some usually hydrogen-rich diamonds, which has defined a defect of great interest, the 1330 nm center; we suggest that this defect, together with the many lines in the 970 to 1000 nm range—referred to as the 990 nm series in this study—are responsible for the complex UV-Vis-NIR spectra seen of these diamonds
Four Pseudo CO2 and seven CO2 diamonds were included in the sampling for this study
Summary
Brown diamonds that owe their color to defects that are not related to post-growth plastic deformation (DR = deformation-related [1], NDR = non-deformation related) are quite rare, in larger sizes (Figure 1); earlier studies of brown diamonds have shown that parcels of melee-sized brown diamonds contained approximately 1.3% of brown diamonds with no signs of significant plastic deformation, designated as CO2 diamonds and Pseudo-CO2 diamonds [2]. CO2 diamonds were first described by Schrauder and Navon based on a single diamond [3]. The IR spectra of such diamonds exhibit absorptions at the approximate positions of where the CO2 ν3 (asymmetric stretch) and the CO2. Pseudo-CO2 diamonds are stones that exhibit optical and spectroscopic properties very similar to CO2 diamonds, but that do not show CO2 absorptions in their IR spectra [2]. The presence and position of the CO2 absorptions were used to suggest that they are caused by pressurized solid CO2 inclusions [3].
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