Large Natural Diamonds with Asteriated Cloud Inclusions
The Carlsbad laboratory recently received for scientific examination two natural diamonds that displayed remarkable light brown cloud inclusions resembling stars (figure 1, left). Such diamonds are sometimes referred to as “asteriated” or “star” diamonds in the literature. Clouds are found in sectors correlating with high hydrogen and nickel concentrations and are thought to consist of graphite inclusions (B. Rondeau et al., “Three historical ‘asteriated’ hydrogen-rich diamonds: Growth history and sector-dependent impurity incorporation,” Diamond and Related Materials, Vol. 13, No. 9, 2004, pp. 1658–1673; A.R. Lang et al., “Is dispersed nickel in natural diamonds associated with cuboid growth sectors in diamonds that exhibit a history of mixed-habit growth?” Journal of Crystal Growth, Vol. 263, No. 1-4, 2004, pp. 575–589; K.V. Smit et al., “Diamond growth from C-H-N-O recycled fluids in the lithosphere: Evidence from CH4 micro-inclusions and δ13C-δ15N-N content in Marange mixed-habit diamonds,” Lithos, Vol. 265, 2016, pp. 68−81).
While asteriated diamonds are occasionally submitted to GIA (Fall 2019 Lab Notes, p. 417; Fall 2019 Gem News International, p. 443), the two recently examined stones were particularly notable for their large size and preserved natural forms. The 8.14 ct diamond was resorbed and maintained most of its rough form on one side, and it was sliced to reveal the underlying inclusion pattern on the other. The preserved rough surfaces also displayed brown radiation stains, indicating their past history of natural alpha irradiation followed by annealing in the earth. The 8.57 ct diamond was a polished stone with bruted edges. In both stones, and consistent with similar asteriated diamonds previously studied, the clouds fluoresced green when subjected to long-wave UV (figure 1, right).
Both diamonds had high hydrogen concentrations, as detected by bulk Fourier-transform infrared spectroscopy. Photoluminescence (PL) mapping using 455 nm laser excitation showed a prominent wide S3 center (496.7 nm), correlating directly with the green fluorescing clouds (figure 2). This peak is associated with nickel (A.M. Zaitsev, Optical Properties of Diamond: A Data Handbook, Springer-Verlag, Berlin, 2001). Other nickel-related peaks in the cloud region were detected in both stones via 633 nm laser excitation at 694.2, 700.5, 787.3, and 793.4 nm (figure 3). Unattributed peaks at 709.1 and 729.6 nm were also detected. Higher peak intensities at 793.5 and 801.8 nm in the cloud inclusion sectors were revealed using 785 nm laser excitation. The former peak is associated with the S2 and S3 centers (Zaitsev, 2001), and the latter is potentially NE8, a nickel-nitrogen complex center (T. Gaebel, “Stable single-proton source in the near infrared,” New Journal of Physics, Vol. 6, 2004, article no. 98).
Both diamonds were analyzed by X-ray computed microtomography (μ-CT), in which the asteriated growth sectors, ~{100}, were visible in three-dimensional reconstructions (figure 4; see video above). The contrast in the μ-CT reconstruction should be mainly a function of the mean atomic number of the material. The asteriated regions more strongly absorb X-rays than the rest of the diamond, an indication that the combined signal from the diamond and inclusions likely had a higher atomic number.
These aesthetically pleasing and scientifically interesting diamonds provide stunning examples of what the natural world has to offer.
