Nonbead-Cultured Pearls from Strombus Gigas
Among the saltwater porcelaneous (non-nacreous) pearls, those of Strombus gigas gastropods are probably the best known among jewelers and collectors because of their vivid to pastel colors ranging from pink to white to brownish. Also known as “pink pearl,” “conch pearl,” or “Queen conch pearl,” this material was used extensively in Art Deco jewelry, and many designers are now incorporating them into modern pieces. Various articles and books have described the extraordinary beauty, use and related traditions, and early attempts at cultivation of pink pearls (see E. Fritsch and E. Misiorowski, “The history and gemology of the queen conch ‘pearls’,” Winter 1987 G&G, pp. 208–231). The typical structure responsible for the sought-after “flame pattern” was detailed extensively a decade ago (S. Kamat et al., “Structural basis for the fracture toughness of the shell of the conch Strombus gigas,” Nature, Vol. 405, No. 6790, 2000, pp. 1036–1040; H. Hänni, “Explaining the flame structure of non-nacreous pearls,” The Australian Gemmologist, Vol. 24, No. 4, 2010, pp. 85–88). Found in the Caribbean region, where Strombus gigas live and are fished for meat, the pearls had no documented cultured counterpart until the work of Hector Acosta-Salmon and Megan Davis at Florida Atlantic University in 2009.
The Laboratoire Français de Gemmologie (LFG) recently had the opportunity to analyze a parcel of eight samples, presented as cultured conch pearls, from a new farm based in Honduras (figure 1). The colors ranged from white and pinkish white to yellowish orange. The shapes varied from oval to baroque, and weights ranged from 0.21 ct (2.7–3.4 × 4.1 mm) to 3.13 ct (6.5–7.6 × 9.3 mm). Their luster was porcelaneous, and the surface was very smooth and homogenous with no defects. Almost all the samples possessed a typical but subtle flame pattern (figure 2).
Initial examination revealed no indication of the culturing process, but X-radiography analysis left no doubt about the material’s origin. The samples’ inner structures revealed a darker central zone, suggesting a less dense material, typical of the graft remainder of the nonbead culture process (figure 3, left). This kind of pattern is not found in the natural conch pearls we have examined in the laboratory (figure 3, right).
According to the farm owners, queen conch pearl culture is undergoing intensive development, and the quality—size, flame pattern, and color—improves with each harvest. While current laboratory techniques are adequate to unambiguously identify nonbead cultured pearls from Strombus gigas, the LFG has established a close working relationship with the pearl farm, paying close attention to possible developments in their distinctive inner structures.