Integrated Chromogenic Analysis of Freshwater Pearls: Revealing the Internal Factors Driving Color Variation

Pearl color serves as the paramount criterion for quality assessment and commercial valuation in the global pearl industry. Freshwater nucleated pearls, which constitute 95% of global production, exhibit striking chromatic diversity. This study deciphers the chromogenic mechanisms of freshwater nucleated cultured pearls in Hyriopsis cumingii from Zhuji, China, through integrated spectroscopic (UV-vis-NIR and Raman), colorimetric (CIELAB), and trace-element (LA-ICP-MS) analyses. We identify polyene compounds as the primary organic chromophores, with C=C bond counts determining core hue: purple (12 C=C bonds), pink (11 C=C bonds), and white/orange (10 C=C bonds). Color expression is further modulated by nacre microstructure; densely aligned aragonite tablets enhance optical interference in purple pearls, whereas irregular tablet arrangements in pink and orange pearls promote diffuse scattering. Crucially, trace elements (Mn, Fe, Cu, Zn) contribute synergistically via metalloporphyrin formation (e.g., Mn-porphyrin in purple variants) and aragonite lattice substitutions. These findings reveal that pearl coloration arises from the interplay of biological factors (organic matrix), physical structure (nacre architecture), and chemical composition (trace elements), providing insights for quality enhancement and sustainable aquaculture practices.