Study on the Structure of Fresh-water Cultured Pearls with Different Body Colors

Four fresh-water cultured pearls with different colors were measured and observed by Raman spectroscopy and high resolution scanning electron microscopy.The results showed that the morphologies of the exterior surface and fracture profile for the four pearls showed different characteristics.The surface of white and yellow pearls presents trim growth steps,but the growth steps of violet and purple pearls are disorder.Especially for the purple pearl,not only the growth steps are irregular,but also the surface owns more bunching steps and defects than others.As for the fracture surface observation,we found that the aragonite platelets thickness of white and yellow pearls was about 200-400 nm wheareas the aragonite lamina of violet and purple pearls ranked more densely and the thickness was about 100-300 nm.After etching by EDTA,it was discovered that there is organic matrix left between the lamina.The violet pearl was the easiest one to be etched by EDTA,and the purple pearl was the most difficult one,so it can be concluded that the purple pearl was very dense.That's why the purple pearl owns the best luster in the situation of bad exterior surface.