Iridescent Botryoidal Growth in Untreated Akoya Pearl
Cultured akoya pearls (Pinctada fucata) are typically admired for their high luster and desirable overtone. Nearly all pearls from this species undergo color treatment before entering the market. One untreated akoya pearl examined by the author exhibited an iridescent and botryoidal subsurface appearance when viewed with a fiber-optic light under microscopic magnification (see above). This was in sharp contrast to the pearl’s appearance without magnification, which showed a light gray bodycolor and smooth surface.
The light gray bodycolor of the pearl was caused by large voids or gaps surrounding its bead nucleus, which could be seen using real-time X-ray microradiography (RTX). Akoya pearls possess very thin nacre platelets that are transparent to translucent when viewed under strong lighting, allowing the viewer to see into the deeper layers of the pearl. The strong iridescence was caused by light interference within alternating layers of aragonite and conchiolin—the two substances that create nacreous pearls and that also have different refractive indices. Specifically, this iridescence is caused when incoming overhead light is reflected from the surfaces between the successive aragonite and conchiolin layers. When the reflected light rays from a bottom layer of nacre interfere with the incoming light, iridescence is created. In pearls, this feature is known as orient (E. Fritsch and G.R. Rossman, “An update on color in gems. Part 3: Colors caused by band gaps and physical phenomena,” Summer 1988 G&G, pp. 81–102). The underlying botryoidal structure combined with the iridescence from the overlying nacre was responsible for the unusual pattern or orient seen in this akoya pearl. This is the first time the author has encountered such a phenomenon in a pearl.
