Mineral Compositions and Organic Color-Related Compounds of Freshwater Bead-Cultured Pearls from Zhuji, Southeast China: Insights from Multi-Spectroscopic Analyses

Freshwater bead-cultured pearls (FWBCPs) from Zhuji, China, have gained significant market prominence due to their large size, unique pearl luster, and diverse color. This study systematically investigated the mineral compositions and organic color-related compounds of twelve representative freshwater cultured pearls through a multi-analytical approach integrating Fourier transform infrared spectroscopy (FTIR), Laser Raman spectroscopy (LRS), ultraviolet–visible spectroscopy (UV-Vis), cathodoluminescence (CL), micro-infrared spectroscopy, and differential thermal–thermogravimetric analysis (TGA-DTA). Key findings reveal that FWBCPs from Zhuji primarily consist of aragonite, organic matter and adsorbed water, occasionally containing vaterite. No obvious correlation was observed between the mineral compositions and the quality of the pearls. Raman spectra exhibit characteristic bands of polyenes near 1525 cm−1 (attributed to the stretching vibration of C=C, ν1) and near 1131 cm−1 (attributed to the stretching vibration of C-C, ν2). The different colors are formed by various polyenes with the exact position of the characteristic bands determined by the concentrations of C=C in the polyenes. FWBCPs and freshwater non-bead-cultured pearls (FWNBCPs) exhibit essentially the same mineral compositions and organic color-related compounds, but can be distinguished from each other based on their internal structures. These results advance the understanding of freshwater pearl formation mechanisms and establish a scientific foundation for quality evaluation in the gemological industry.