Unveiling Distinct Gemological and Microstructural Features of Freshwater Cultured Pearls With Novel Composite Nuclei

A type of freshwater pearl, measuring approximately 6 mm in diameter, was systematically investigated using multiple complementary techniques, including optical spectroscopy, X-ray-based methods, thermal analysis, and scanning electron microscopy (SEM). The nuclei were classified into type I (composed primarily of barite and organic matter) and type II (composed mainly of calcite and organic matter). The resulting nacre was robust, with a thickness of approximately 0.76-1.00 mm, exhibited periodic tree-ring-like Mn/Sr bands with consistent enrichment of phosphorus. Interestingly, SEM observations revealed that, in the radial direction from the inner to the outer part of the nacre, a transition zone with a thickness of approximately 20 μm existed near the nucleus, where the structure gradually transitioned from a microstructurally disordered morphology through a quasi-tablet region to a regular aragonite platelet organization. This study provides a theoretical foundation for the commercial cultivation of innovative pearls using novel nucleus materials and establishes a technical framework for their rapid, accurate, and non-destructive identification. Most importantly, the observed mineralization features within the nacre adjacent to the nucleus offer novel insights into fundamental biomineralization processes and may inspire the biomimetic design of nacre-like materials with superior mechanical properties.