![]() Electron microscopic study of the nanometer minerals and microstructure of marine cultured pearls. Characteristics of nuclei in Chinese freshwater cultured pearls. The authors declare no conflict of interest. Porosity of the nacre layer was analyzed by μ-CT scanner (nanoVoxel 1000) and its analysis software, with a spatial resolution of 5 μm. The value of surface height unevenness evaluation parameters Sa (the standard deviation of the height distribution) and Sq (arithmetic mean of height difference) were obtained through its analysis software. Laser scanning confocal microscope OLS5000-SAF was used to observe the surface appearance features and calculate roughness parameters, with the resolution of 1 nm and a 405 nm laser light source. ![]() The surface reflectivity of the pearl samples was calculated by the ratio of the test lightness value and reference value. The brightest spot with 1mm diameter of each sample was selected and measured three times as test values. The lightness value of incident light was measured and recorded for reference before the sample test. The luster of pearl samples was measured by chroma meter CS-200 under D65 illuminant (simulating sunlight) and reflected by the surface reflectivity. The influence of the nacre layer’s porosity on the visual appearance of Chinese large freshwater nucleated pearls, which determines the quality grade, needs deeper investigation. Nevertheless, pores can be created during the formation of pearls and lead to a change in microstructure. Some investigations also discussed the luster formation from the aspect of chemical composition and microstructure. Previous studies mainly focused on the characteristics and formation mechanism of the microstructure in other different varieties of pearls. However, regarding the new emerging Chinese large freshwater nucleated pearl, the intrinsic incentives to form different appearance quality characteristics and the influence of internal structure on both the luster and surface roughness deserve to be discussed and analyzed. Luster and surface roughness are two important indicators of the grading and evaluation of pearl appearance quality. Related results can provide reference for the scientific and quantitative evaluation of pearl quality. It was found that the porosity of the nacre layer had a significant influence on appearance features, with an increase of porosity showing more surface blemishes (higher surface roughness parameters) and weaker luster (lower reflectivity). ![]() ![]() In this study, light reflectivity, surface height unevenness parameters and porosity of the nacre layer were measured by chroma meter, laser scanning confocal microscope and X-ray computed tomography (μ-CT), which quantitatively described the characteristics of luster, surface roughness and structure compactness of the nacre layer. However, its quality evaluation mostly depends on appearance observations, and the influence of the nacre layer’s internal microstructure on the gemstone’s appearance needs further investigation. The Chinese large freshwater nucleated pearl has become popular for its unique appearance throughout the international jewelry market in recent years. ![]()
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