Micro-World Gems & Gemology, Winter 2021, Vol. 57, No. 4

Natural Black Coral with Polyp Structure

Amy Cooper and Britni LeCroy

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Diagnostic polyp structure of black coral.
Figure 1. The surface of the black oval cabochon displayed a vivid example of the diagnostic polyp structure in black coral, shown using reflected light. Photomicrograph by Britni LeCroy; field of view 2.90 mm.

The authors recently examined a large black oval cabochon that was opaque with a slight waxy luster. Microscopic examination revealed a layered concentric tree-like structure when a fiber-optic light was used, as well as round ring-like spherical structures. These observations are consistent with coral, and these individual spheres are referred to as polyps (figure 1).

Intense orangy brown and black polyp structure consistent with natural black coral.
Figure 2. Using oblique fiber-optic illumination, the orangy brown color and polyp structure were clearly visible. Photomicrograph by Britni LeCroy; field of view 2.90 mm.

This is a great example of the diagnostic surface structure produced when black coral is cut and polished into a cabochon. Oblique fiber-optic lighting revealed an intense orangy brown and black pattern, also consistent with the polyp structure of natural black coral (figure 2). This was the clearest example of this polyp structure the authors have examined in black coral. For additional information and images on black coral, see E.W.T. Cooper et al., Guide to the Identification of Precious and Semi-precious Corals: In Commercial Trade, World Wildlife Fund, Vancouver, Canada, 2011.

Amy Cooper is a senior staff gemologist, and Britni LeCroy is a staff gemologist, at GIA in Carlsbad, California.