How Shell Pearl as a Pearl Imitation ？ GIA How to Say it.

Imitation pearls made of shell, or “shell pearls,” have a long history in the jewelry market and have been reported in previous Lab Notes columns (Fall 1984, p. 170; Winter 1986, p. 239; Summer 2001, pp. 135–136; Summer 2004, p. 178). The shell beads are often coated with artificial materials to simulate a wide variety of natural and cultured pearls in the market. Recent submissions of such imitations to GIA prompted researchers in the lab to obtain several samples from a commercial website. Labeled as shell pearls (figure 1), they resembled Tahitian black, white, and yellow/golden South Sea cultured pearls.

Although the samples were similar in appearance and heft to cultured pearls, routine gemological tests revealed unnatural surface characteristics. The black shell pearl necklace also exhibited multicolored but rather oily orient. Magnification showed numerous minute particles with a glittering effect (figure 2, left), as well as a lack of the obvious overlapping nacre platelets commonly found in nacreous pearls. These features suggested that an artificial coating had been applied to the bead. Inspection near the drill holes of some samples revealed chipping and peeling of this very thin coating, exposing white bead-like shell material underneath (figure 2, right).

X-radiography showed only the inner bead, with occasional parallel banding and cracks—similar to the banding and trematode “tunnels” common in some saltwater shell beads—while the outer coating remained transparent (figure 3). EDXRF analysis detected a high concentration of bismuth, which usually does not occur in pearls but had been reported previously in coated natural and cultured pearls (Fall 2005 Gem News International, pp. 272–273; Winter 2011 Lab Notes, pp. 313–314). Finally, Raman spectroscopic analysis of the exposed white bead identified it as aragonite. These results, coupled with the bead’s inert reaction under X-ray fluorescence, confirmed it was made of saltwater shell.

We have noted the use of the terms shell pearl or just pearl for such imitations in product descriptions on numerous commercial websites. Such inappropriate nomenclature could be misleading to inexperienced buyers. According to the CIBJO Blue Book on pearls, “Imitations or simulants of natural pearls and cultured pearls . . . shall be immediately preceded by the word ‘imitation’ or ‘simulated’, with equal emphasis and prominence . . . as those of the name itself.” Imitation pearl would be the proper name of this material, and consumers need to be aware of its true identity.

the post from GIA ,  https://www.gia.edu/gems-gemology/summer-2014-labnotes-shell-pearl