Micro-World Gems & Gemology, Spring 2022, Vol. 58, No. 1

Acicular Troughs Coincident with Green Stains on Rough Diamond Surface

Diamond from Guyana exhibits green skin.
Figure 1. A ~0.12 ct green-skinned diamond from Guyana. Photo by Roy Bassoo.

The author acquired an alluvial rough diamond from Guyana showing unique surficial features that do not appear to have been documented elsewhere. The specimen was a twinned crystal consisting of two intergrown octahedral crystals, ~0.12 ct in weight, with numerous flat-bottomed hexagonal etch features and trigons and a nearly pervasive green skin (figure 1). Most interesting were fine, linear, and randomly oriented features on the diamond surface that were spatially associated with the green stains (figure 2). Radiation stains are known to cause localized expansion in diamond (L. Nasdala et al., “Radio-coloration of diamond: A spectroscopic study,” Contributions to Mineralogy and Petrology, Vol. 165, 2013, pp. 843–861; S.C. Eaton-Magaña and K.S. Moe, “Temperature effects on radiation stains in natural diamonds,” Diamond and Related Materials, Vol. 64, 2016, pp. 130–142). These features also cross-cut trigons, implying that they post-date the diamond’s growth in the earth’s mantle and its surface dissolution within the kimberlite magma (figure 3). These features may be the result of localized dissolution caused by proximal acicular, radioactive, and as yet unknown minerals, leaving behind a cast impression. Green color in diamond can be produced by natural irradiation of the crystal lattice by exposure to radiation from proximal and adjacent radioactive grains (e.g., zircon, monzonite, uraninite, and K-feldspar) or salts dissolved in percolating fluids surrounding the diamond (C.M. Breeding et al., “Natural-color green diamonds: A beautiful conundrum,” Spring 2018 G&G, pp. 2–27). Gamma, alpha, or beta radiation emanating from these sources are thought to cause collections of defects and impurities within the diamond. Gamma and beta radiation have low energies and long penetration depths producing green bodycolors. Due to its high energy, alpha radiation has the shortest penetration depth and only within the first 25 μm of the diamond surface, creating green skins, stains, and spots.

Green radiation stains on an unpolished diamond.
Figure 2. Left: A magnified view of green radiation stains on an unpolished diamond surface in transmitted light. Right: An image of the same surface in reflected light with the radiation staining (outlined in red) and preserved acicular troughs. Photomicrographs by Roy Bassoo; field of view 0.5 mm.
Magnified view of green radiation stain and acicular troughs.
Figure 3. A further magnified view of the green radiation stains on an unpolished diamond surface in transmitted light (left) and an image of the same surface in reflected light with the radiation staining outlined in red and preserved acicular troughs which cross-cut a flat-bottomed trigon (right). Photomicrographs by Roy Bassoo; field of view 0.25 mm.

Although unusual, radiation-related surface features have been previously documented (e.g., Spring 2021 G&G Micro-World, pp. 66–67). The ones seen in this diamond from Guyana appear to be unique, however. It is quite likely that these features are pervasive in placer diamonds but have not been studied in detail.

Roy Bassoo is a postdoc research associate at GIA in Carlsbad, California.