Spectrum and Chemical Composition Characteristics between White "Freshwater Akoya Pearls" and Akoya Pearls

In recent years, a kind of high-quality white freshwater nucleated pearls in small size (3-10 mm) has appeared in pearl market in P. R. China, which is commercially called "freshwater Akoya pearl" (F-Akoya) because of its similar appearance to white Akoya pearl (Akoya).In this paper, F-Akoya and Akoya pearl samples from Zhuji market were collected and analyzed by conventional gemmological test, photoluminescence (PL) spectrometer, three-dimensional fluorescence (3D FR) spectrometer and LA-ICP-MS. Although F-Akoya pearls are similar to Akoya pearls in appearance, they are clearly different in the following aspects: (1) In terms of the thickness and morphology of the nacreous layer, the Akoya is thinner with fish-scale-like grains and fold-like flaws on its surface, while the F-Akoya is thicker without fish-scales-like grains; (2) In terms of the F/A value, namely, the ratio of the overall fluorescence intensity (F) of the PL spectra to the main peak intensity of aragonite (A), the F-Akoya usually ranges in 1 to 5, much lower than the F/A range of Akoya(13 to 169); (3)In terms of the luminescence center Ex/Em in the 3D FR spectra, the F-Akoya ranges in 350~380 nm/420~440 nm with high fluorescence intensity, while the Akoya ranges in 350~400 nm/430~480 nm with low fluorescence intensity; (4) In terms of minor elements shown by LA-ICP-MS, F-Akoya is obviously rich in Mn, while Akoya richs in Na, Mg, K, Sr; (5) In terms of the rare earth element pattern, F-Akoya is enriched in LREE, while Akoya shows the opposite. The composition analyses revealed that the composition of the F-Akoya and Akoya depends on their growth water environment.Importantly, they can be distinguished by the ternary component diagrams and REE partition pattern diagram.