Gray-to-Blue-to-Violet Hydrogren-Rich Diamonds From The Argyle Mine, Australia
The Argyle diamond mine is the only known source of type laB hydrogen- and nitrogen-rich diamonds colored gray to blue to violet. Twenty such diamonds were studied to investigate the relationships between their spectroscopic characteristics and color grades. The unusual color is the result of broad absorption bands centered at ~520-565 nm feature. Spectral deconvolution of this feature suggests that is may be a composite, including H-related absorptions at ~545 and 563 nm and bands at 520-530 and 551 nm. The near- and mid-IR regions exhibit strong absorptions, including those related to H and N, many of which become more intense with increasing color saturation. The PL spectra exhibit peaks associated with nickel-related defects, which may play an important role in the coloration of the more violet diamonds in this group. H-rich gray-to-blue-to-violet diamonds, which are not known to be treated, can be readily separated from similar-hued diamonds that may be HPHT enhanced or synthetic.
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HGBV diamonds exhibited different types of characteristic internal features. These included (left, magnified 52x) coarsely textured channels, where chemical etching occured along preexisting cracks or cleavages; they can sometimes be very deep, as seen in the 0.72 ct stone in the center (magnified 40x). Another type of feature (right, magnified 52x) either etch or growth tubes, consists of hollow, icicle-like acicular inclusions. Photomicrographs by C.P. Smith.
