The gemological characteristics of yellow seawater bead-cultured pearl farming in Beihai City, southern China

Abstract

In this study, we comparatively investigated the gemological and fine-microstructural characteristics of yellow seawater pearls cultured in Pinctada fucata martensii using X-ray computed microtomography (μ-CT) analysis, ultraviolet-visible (UV−Vis) reflectance spectroscopy, field emission scanning electron microscope (FE-SEM), Raman and photoluminescence (PL) spectroscopy, and Laser ablation−inductively coupled plasma−mass spectrometry (LA−ICP−MS). Detection of the nacre thickness indicated that the average nacre thickness varied from 0.17 to 1.35 mm, and further theoretical calculations showed that the nacre accounted for about 15–68% of the whole pearl by volume. For yellow or light-yellow pearls, the spectroscopy results showed characteristic patterns such as certain distinct absorption features around 360, 400, 430, and 450 nm in the UV−Vis reflection spectra and the organic pigment-related peaks at approximately 1130 and 1530 cm^–1 in the Raman spectra. Furthermore, dominant aragonite peak intensities were observed in the PL spectra of these samples, with the ratio between the overall fluorescence intensity (F) and the height of the main aragonite peak (A) consistently below 5. SEM observations showed that multiple amorphous structures, mainly composed of aragonite, coexisted in the nacre in these portions of the pearls, one of which was closely adjacent to the nucleus of the pearl. In addition, LA-ICP-MS analysis clearly revealed the nacre of the outer saltwater layers and the inner freshwater nucleus. Interestingly, we directly observed that the quantities of Na and Mg in the nacre were higher than that those in the freshwater nucleus. These findings substantially enhanced our knowledge of gemological, mineralogical, and fine-microstructural characteristics of as-grown yellow seawater pearls. Most importantly, this study revealed some key characteristics that could be used to identify imitation and color-treated yellow pearls and further guide the pearl production.