Microscopic morphology of prismatic layers of freshwater cultured pearl

The microscopic morphology of the internal structure of freshwater cultured pearls with different colors were investigated by X-ray diffraction (XRD) and field emission-scanning electronic microscope (FE-SEM)．Results clearly reveal that aragonite is found in high quality freshwater cultured pearls, but calcite and vaterite are not found, and the thickness of aragonite sheet in nacreous layer is not the same to pearls with different colors. The prismatic layer is near to the nucleus of the high quality pearls, which is made of a number of cylindrical or banded aragonite fibers. The fibers are oriented nearly parallel to the aragonite sheet. In addition, it is firstly observed that the aragonite fiber and sheet appear alternately in prismatic layer. Meanwhile, a modified model of the internal structure of freshwater cultured pearls is firstly established.