Comparative Study Of Nucleated And Non-nuclear Freshwater Pearls

China is a big country in freshwater cultured pearls,and the annual output of freshwater pearls ranks first in the world.In recent years,a new variety of freshwater nucleated cultured pearls has emerged in the jewelry market and has quickly attracted widespread attention.However,there are few studies about the differences between freshwater nucleated and non-nuclear pearls.In this thesis,gemological features are observed by microscopic magnifiers,Diamond ViewTM,Refractometers and other tools.Comparative studies were also carried out by XRD（X-Ray diffraction）,Electron probes,SEM（Scanning electron microscope）and other modern test instruments.The following conclusions were obtained:Freshwater nucleated pearls have larger volume and higher color saturation than non-nuclear pearls.In the density test,the pearl layer’s density of the nucleated pearl is smaller than that of the non-nuclear pearl.In addition,freshwater non-nuclear pearls may have samples that do not have pearl luster totally.Nucleated pearls do not see this feature,but there are sporadic distributions of matte plaques.The SEM image of the pearl shows that the freshwater nucleated pearls of different body colors have thicker aragonite plate than the non-nuclear pearl.In the test sample,inner aragonite plate with strong pearl luster in the purple non-nuclear pearl is arranged very irregularly,and this support the uniform arrangement of the inner aragonite is not the only factor that affected the pearl’s luster.For the observation of the whole non-light bead in the non-nuclear pearl,it seems that the absence of the light bead is caused by the extremely irregular arrangement of the inner aragonite plate,and the light scattering of the semi-self-shaped aragonite plate.When the matte plaque of the nuclear pearl GC-1 was observed,some fibrous,long plate-like substance was exposed,with many semi-self-shaped aragonite plates around.It is deduced that the matte plaques of the nucleated pearls is not caused by the observed long-plate flaky deposits,but by the light scattering of the internal dispersed spherical matter.The vibrational peaks of aragonite carbonate can be found in 699,711,877,1074,1488,1780 cm^-11 of Infrared spectroscopy.Organic matter-related absorption peaks are shown at 2981,2925,2846,2524,1647 cm^-1.The absorption peak of 1467,1496 cm^-1are suspected to be vaterite,appeared in the GC-1 sample.Subsequent XRD experiments also measured the characteristic peak of the vaterite phase in the GC-1sample.Raman spectroscopy showed that the white sample showed no vibration after1000 cm^-1,the orange sample had vibration near 1135 and 1526 cm^-1,and the purple sample had vibration near 1125 and 1510 cm^-1.The vibration peaks of orange and purples’are caused by polyacetylene organic matter,which is closely related to the color of pearls.