Study On Gemmological Characteristics Of High-Quality Fresh Water Cultured Pearls Modified By Color Using Physical Method

Back in the1950s, the Japanese started to conduct experiment of using y-ray irradiation on the pearls to change their colors, but the resulting color is relatively very single. In recent years, a great number of experts in our country have been engaged in this experiment. Due to the low cost, the stability of changed color and non-residual radiation hazards, this method of changing pearl’s color by y-ray irradiation has been attached great importance. At present, however, because our domestic technique in this field is still immature, the color of the pearls after experiment is gray and the gloss is not satisfactory. As a result, the finished product of this kind on the market is very few. Our research group successfully carried out irradiation to change the color of high-quality fresh water pearl and get some samples with satisfactory color and gloss. Nevertheless, the mechanism of color change and the gemological characteristics of the pearls after color change are still not clear. We take domestic high-quality freshwater cultured pearls and black pearls after irradiation as the research object by using a series of methods such as Infrared Spectroscopy(IR), X-ray Diffraction(XRD), Inductive Coupled Plasma Emission Spectrometer(ICP-AES), Atomic Force Microscopy(AFM), Field Emission Scanning Electron Microscopy(FESEM), High Resolution Transmission Electron Microscopy(HRTEM) and so on. After a careful comparison and analysis on the organic content, inorganic elements as well as surface structure of the pearls before and after irradiation, we get the following conclusions:1. Pearl contrast infrared spectra of irradiated and know the composition of high-quality fresh water pearl aragonite calcium carbonate, organic matter and adsorbed water, and others did not find that the phase composition MnCO3. In the organic range2919cm-1～2342cm-1range after the change color with the weakening of the absorption spectrum, and some disappeared, suggesting that physical methods to color the pearl coloring mechanism of irradiation and organic chemical change.2. By the XRD and IR, we can see both the aragonite CaCO3, MnCO3did not find the existence of the physical laws of the previous view that change color pearl coloring mechanism is due to trace MnC03in the y-ray irradiation or oxidation Mn2O3MnO2statement due to be questioned.3. According to the FESEM analysis, the aragonite chips of pearl layers do not changed significantly after the radiation. But the aragonites plates of the pearl surface before irradiation are similar to the rounded hills tactile so that diffuse reflection is generated. While the aragonites plates of the pearl surface after irradiation become relatively smooth so that glass reflection is generated. This is a very good explanation of why the luster of the pearl after color change is much brighter than before the color change. 4. The research of AFM and HRTEM shows that the peal layer is natural nanometer material (5～80nm). The aragonite nanometer grains constitute aragonite micrometer chips which in turn constitute aragonite layer and the process goes on until the final peal layer is formed. The macro noticeable color difference does not show any clear change in the ultra-structure. The research result believes that the color of pearls and pearl surface ultra-structure, namely the nanometer-size aragonite have no significant correlation.