Lizard Skin on Deformed Diamond
Facets that are nearly parallel to a diamond’s octahedral crystal plane often develop a wavy, rippled appearance called “lizard skin” during polishing (e.g., J.I. Koivula, The MicroWorld of Diamonds, Gemworld International, Northbrook, Illinois, 2000, p. 63). The term is also used more broadly to describe any bumpy, uneven surface texture that develops on polished diamond facets. It is often attributed to polishing off-grain. Recently, GIA’s New York lab encountered a 2.67 ct type IIa diamond (figure 1, left) with especially prominent lizard skin texture on multiple facets (figure 1, center and right). In this case, the texture appears to have developed due to a preexisting deformation fabric or structure inherent to the diamond itself, rather than merely as a consequence of poor polishing technique.
The surface texture has a pattern to it, with small bumps appearing to line up in corridors. Some corridors clearly continue from one facet to the next (figure 1, right). The continuity across facets suggests that this texture is the surface expression of irregularities that actually extend into the volume of the diamond itself. At high magnification with crossed polarizing filters, the lizard skin can be seen to conform to the shape and texture of the strain pattern within the diamond—that is, the pattern of bumps and depressions matches up where the intricate internal dark/light pattern of tatami birefringence meets the polished surface. This connection supports the idea that the surface texture is a reflection of underlying crystal imperfections.
Unusual hydrogen- and methane-bearing metallic inclusions in this diamond suggest it originated from extreme depths of about 360–750 km in the earth’s mantle (E.M. Smith et al., “Large gem diamonds from metallic liquid in Earth’s deep mantle,” Science, Vol. 354, No. 6318, 2016, pp. 1403–1405). In this high-temperature environment, deformation and annealing over a long period of time may have given the diamond a mosaic crystal structure. This natural phenomenon occurs when heat allows the crystal to undergo recovery, a process of dislocation reorganization that divides the distorted grain into a mosaic of smaller, undistorted subgrains without new crystal growth. The subgrains, measuring about 10–30 microns, are like bricks in a wall; while the wall may be slightly curved, the individual bricks are not distorted. These distortions become localized as subgrain boundaries rather than spreading continuously through the crystal. As a result, the subgrains will be oriented in slightly different directions.
Because polishing is so strongly dependent on crystal orientation, if a single facet consists of a mosaic of subgrains with varying orientation, polishing will be uneven. The resulting surface may develop a pattern of bumps and depressions that mimics the size and shape of underlying subgrains and, in turn, reflects the diamond’s natural deformation history. Samples like this one are interesting for scientists because little is known about the geological conditions that lead to the various deformation-related features seen in natural diamonds.