Vaterite found in saltwater natural pearls from Pinctada Sp. mollusks

In this study, we report the detection of vaterite in natural saltwater pearls reportedly found inside Pinctada species mollusks, collected from the pearl oyster beds near the water of Kuwait in the Persian (or Arabian) Gulf. This rare polymorph of calcium carbonate was found on both surfaces and/or cross-sectional areas. Raman spectroscopy and X-ray diffraction (XRD) analyses were carried out in order to characterize the structures, which confirmed the presence of vaterite. The results of one pearl are detailed in this article, which showed a predominantly vaterite formation with a calcite central core. Our results showed that vaterite not only existed in freshwater cultured pearls grown in freshwater mussels as previously reported but also can be found in pearls from marine bivalve mollusks. To the best of our knowledge, this is the first time vaterite was conclusively identified in saltwater pearls and pearls of natural origin, and such information provided invaluable insights into biomineralization of calcium carbonate in these unique biogenic gem materials.