The color of most black diamonds is attributed to graphite micro-inclusions or metallic inclusions (S.V. Titkov et al., “An investigation into the cause of color in natural black diamonds from Siberia,” Fall 2003 G&G, pp. 200–209), and it is rare to see dense clouds of micro-inclusions producing a black appearance (see Spring 2007 Lab Notes, pp. 52–53). Since November 2016, the National Gemstone Testing Center (NGTC) in Shanghai has routinely received large quantities of polished black diamonds. One such stone, a 1.03 ct opaque round brilliant (figure 1) that we believed to have been heat treated, had a striking pattern of micro-inclusions we had never seen before.
Microscopic observation revealed a “double cross” pattern arising from an abundance of pinpoint-like gray inclusions restricted to certain sectors of the diamond (figure 2). The outer black cross was oriented in the same direction as the inner pale cross. The inclusions’ crystalline forms were too small to resolve at 40× magnification. These small inclusions absorbed a large proportion of the light entering the diamond, causing the black appearance. The FTIR absorption spectrum confirmed the diamond was type IaAB and H-rich, with nitrogen-related absorption from 1000 to 1500 cm–1, a graphite-related peak at 1582 cm–1, and H-related IR peaks at 1405, 2785, 3050, 3107, 3154, 3236, 4169, and 4496 cm–1. We confirmed that the pinpoint-like inclusions were graphitized hydrogen clouds (see P. Johnson et al., “Hydrogen rich treated black diamonds,” poster presentation at Geological Society of America 2016 annual meeting).
When observed in the high-energy short-wave UV radiation of the DiamondView (figure 3), the bulk of this black diamond displayed moderate yellowish green fluorescence. Four small areas were basically inert to UV radiation; these also corresponded to the dark cross.
The unique double cross pattern and the DiamondView image revealed an interesting growth structure. The diamond apparently underwent two distinct phases of growth, with each phase forming a cross developing from separate mixed-habit growth with contemporaneous cuboid and octahedral sectors. The well-developed cuboid growth sectors contained regions with cloud-like light scattering that caused the black parts; very poorly developed octahedral growth sectors with a small amount of inclusions formed the light cross. The cuboid growth sectors exhibited yellowish green fluorescence, while the inclusion-poor octahedral sectors were inert to the short-wave UV light.
The interesting pattern of inclusions and distinctive growth features of black diamonds such as this one needs more attention.
Contributors’ note: This study was supported in part by the National Natural Science Foundation of China (grants 41473030 and 41272086).