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 (Ca_0.53Mg_0.47CO₃) 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.

A small fragment (~20 μm) was separated from the center of the pearl and mounted on a micro loop for XRD data collection. The data were collected on a 3 circle single-crystal X-ray diffractometer, which is equipped with a Rigaku 007 Cu-Kα X-ray source and a Bruker PROTEUM-F135 CCD detector. Three φ+ω-rotation frames were collected for 600s/frame with a detector distance of 150 mm and 2θ=30°, 50°, 70°. The data were reduced to 1D powder diffraction pattern using APEX3 software. The crystal structure is refined using the Rietveld method against the integrated data in the Jana2006 program.

Chemical analyses were performed using a Thermo Fisher Scientific’s iCAP Qc ICP-MS, coupled with an Elemental Scientific Lasers NWR213 laser ablation system with a frequency quintupled Nd:YAG laser operated in Q-switched (pulsed) mode at a wavelength of 213nm and pulse duration of 4 ns. Detailed operation conditions for the laser and the ICP-MS instrument are the same as what was described in the previously published paper. The chemical composition was initially internally standardized with 43Ca at a preset value of 400400 ppmw calculated and assumed from pure calcium carbonate. The data was then normalized and converted to 100 wt. % oxides based on 1 carbon and 3 oxygen atoms.

A Renishaw inVia Reflex micro-Raman spectrometer system with a 50× magnification Leica objective lens and an 830 nm diode laser excitation wavelength at room temperature was used to analyze structural characteristics on various locations of the sample in a scanning range from 100 to 1600 cm^–1. It is a common technique used to distinguish CaCO₃ polymorphs (mainly between aragonite and calcite) with reference to different band positions of carbonate ion (CO₃²⁻) modes.