Comparative Study Of Freshwater And Seawater Nucleated Cultured Pearls

The essay discusses similarities and differences of properties, structures and appearance between the Chinese freshwater nucleated pearls and some of the seawater nucleated pearls such as the Japan Akoya pearls, the white South Seas pearls, and the Tahiti black pearls. This is done by using various microscopes as well as techniques, FITR, Raman Spectrum, XRD, Cathode Luminescence, Diamond ViewTM, to name a few.The structure of seawater and freshwater nucleated pearls include a pearl layer, a prismatic layer, an organic layer and a pearl nucleus. The pearl layer of the seawater nucleated pearls consists of aragonite pieces that are slightly protruding as “beach line” and “contour line”. The aragonites can also be pentagon and heptagon shaped, instead of hexagon shaped. The prismatic layer usually exists near the nucleus, while growing alternatively with several layers of aragonite pieces. This property is known as the “prism-flake” structure. The prismatic layers of freshwater pearls can also exist near the surface. The organic layers usually exist at the intersection of the nucleus and the pearl layer. The seawater pearls may appear at the intermediate layer between the pearl layer and the organic layer. The existence of the prismatic layers does not imply the species or the quality of pearls. However, the shape and the arrangement of the aragonite pieces may represent the the quality: similarity in shape and tight arrangement of the aragonite suggest better quality of the pearl.The main component of both species is aragonite. They can also consist of organic matter and water. The FITR shows 1083, 861, 1483, 713, 699, 1788 cm-1 of carbonate vibration in aragonite; 2981, 2918, 2853, 2625, 2521, 1649 cm-1 of organic matter absorption peaks; and 3392-3367cm-1 of water absorption. The Raman Spectrum shows 1083, 703 cm-1 of carbonate vibration in aragonite, 284, 272, 247, 212, 206, 190, 179, 152, 142 cm-1 aragonite lattice vibration. The XRD shows strong(002) interplanar diffraction peak intensity, where the Akoya and the freshwater pearls demonstrate high intensity.Vaterite is found on the surface and inside the freshwater nucleated pearls, in the shape of lath, pellet and patch-like. The FITR shows 879, 1498, 1468cm-1 characteristic peaks of Vaterite, and 1460, 1462cm-1with the Raman Spectrum. Vaterite in pearls has little light reflection and becomes bright red in cathode luminescence and is categorized as slightly lower than the surface layer with littlecrystal morphology in SEM. The occurrence of Vaterite on the surface of the pearls can cause a negative impact on the quality of the pearls.The pink(partial orange) freshwater nucleated pearls show 1527, 1130, 1296, 1008~1020cm-1 carotenoid peaks with Ramen Spectrum, while purple ones show 507, 1120cm-1 organic matter peaks. The more intense the color is, the more obvious the peaks are. The results can be summarized as carotenoid causes the colors of pink and orange in the freshwater nucleated pearls.