SPECTROSCOPIC FEATURES OF OUTER GREEN FLUORESCENCE RIMS IN PINK ARGYLE ROUGH DIAMONDS

Diamonds can emit fluorescence when exposed to incident electromagnetic radiation, the color of which is affected by optical defects in the diamond lattice. Typically, fluorescence color remains fairly consistent between the rough outer surface of the diamond and the interior, if there is not significant growth zonation or post-growth treatment. However, the authors have observed that many pink diamonds from Argyle mine, Australia display predominantly green fluorescence while in their rough form, and blue fluorescence when this outer layer is removed upon faceting. Seven pink diamond offcuts were obtained, all of which maintained both green fluorescing rough surfaces and blue fluorescing cleaved surfaces. Bulk Vis-NIR absorption detects that all of the samples are colored by the 550-nm absorption band, and bulk FTIR determines the diamond type to be Type IaA<B, as is typical for Argyle pink diamonds. One sample was parallel plated and photoluminescence (PL) mapped using 455 nm laser excitation, revealing higher intensities of H3 ((N-V-N)⁰) and H4 defects (4N+ 2V) at the rough surface, both of which cause green fluorescence in diamond, as well as the radiation-related TR12 defect. Cathodoluminescence imaging and spectroscopy estimate the thickness of this green fluorescence “layer” as ~6-19 µm, within the maximum depth of penetration of alpha radiation into diamond. Characterization of an end piece of this sample using 375 nm laser excitation using a newly developed 3D hyperspectral photoluminescence imaging microscope confirms that there is higher H4 intensity at the surface of the diamond in comparison to the N3 defect, the source of blue fluorescence. Photoluminescence mapping using 532 nm laser excitation detects higher intensities of the N-V⁰, N-V^- and 588.0 nm defects near the surface of the rough diamond as well. Spot PL on the green fluorescing rough surface and the cleaved interior of an additional diamond detected higher H3, H4, TR12, and N-V center intensities on the rough surface, consistent with the other sample. The combination of such defects indicates that these diamonds must have experienced both irradiation and annealing. The depth of penetration of the fluorescence layer suggests the samples experienced alpha irradiation, which is unusual as they do not display any radiation stains.