I n the Polynesian language, pipi means "small" or "tiny" and is used to refer to the small, predominantly yellow or "golden" pearls that originate from the bivalve mollusk Pinctada maculata (Gould, 1846). As the smallest mollusk species in the Pinctada genus, Pinctada maculata rarely exceeds 5 cm when measured in the anterioposterior or dorsoventral positions. The mollusk lives in the Indo-Pacific Ocean, mostly around French Polynesia and the Cook Islands, where they are often found in association with the Pinctada margaritifera mollusk species (Strack, 2006). Pearls from Pinctada maculata often form in round to near-round shapes. As the name implies, the small shells produce small pearls that rarely exceed 8 mm in diameter (Krzemnicki, 2014). Based on GIA's experience, 6 mm or under is more typical of the species. Pipi pearls are known to occur as natural pearls rather than cultured and, compared with other Pinctada species, are deemed rare. One report recorded only one gem-quality pearl found from a total of 355 mollusks (Passfield, 1997). In 1950, several cultured pearl experiments using Pinctada maculata reportedly took place but were unsuccessful (Segura et al., 2014). In the late 1990s, a few Pinctada maculata cultured blister pearls resulted from experiments in the waters off Penrhyn, an island in the northern atoll of the Cook Islands (Kessrapong et al., 2017). The nacre covering the bead nuclei did not fully overgrow the nuclei, however, and this attempt was not very successful. Some reports suggest that the Pinctada maculata mollusk is not abundant enough for commercial exploitation. Local regulations also protect them from being harvested by foreigners, further limiting the chances of cultured pearl production (Buscher, 1999). Given the limited documentation of Pinctada maculata cultured pearls, it is almost certain that all pearls currently fished from the species are natural pearls.

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Natural pearls are produced without human intervention, mainly due to various irritations from the surrounding environment to their mantle tissues. Pearls usually possess similar mineral compositions to the host shells, which means they are also dominated by aragonite and calcite. In this study, we report a natural pearl from a Cassis species mollusk containing granular central structures. Raman spectroscopy, laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS), energy dispersive X-ray spectroscopy (EDS) coupled with scanning electron microscope (SEM), and X-ray diffraction (XRD) analyses were carried out in order to characterize the mineral composition in the center region of this pearl. Our results showed that this pearl’s center was made of mostly disordered dolomite (Ca0.53Mg0.47CO3) mixing with small amount of aragonite and high magnesium-calcite. To the best of our knowledge, this is the first time disordered dolomite was conclusively identified inside of a natural pearl and such information expanded our knowledge on internal growth structures and formation of natural pearls.