Investigation on the Coloring Mechanism of Freshwater Cultured Pearls with Different Color

The coloring mechanism of pearls was debated on organic pigment or trace metal ions. In view of the theory of the coloring mechanism from organic pigment, which had no relationship with the structure of the object combined with the results of the trace metal elements determined by X-ray Fluorescence Spectrometry or the other instruments, the above two theories can not explain the coloring of pearls. The coloring mechanism of freshwater cultured pearls was investigated by Fourier Transform Infrared Spectroscopy, X-ray Fluorescence Spectrometry and Field Emission-Scanning Electronic Microscope. The results indicate that, in terms of pearls with different color, the characteristic peaks were almost the same in the middle IR spectra of freshwater cultured pearls, which is due to the chemical bonds of organics in pearls. The contents of Ti, Fe, Mg and Cu elements in pink pearls were higher than those in white and purple pearls. Additionally, the content of Mn in white pearl was higher than that in pink ones. The contents of Ti, Fe and Cu elements were different between white and pink pearls, but they were the same between white and purple pearls. The color of ground powders of white, violet and pink pearls was almost the same yellow and the domain reflection wavelength was (582±1) nm. The above results showed that the coloring mechanism is from organic pigment or those trace metal elements directly. Although the diameters of pearls were the same, it was firstly discovered that the thickness of aragonite sheets were obviously different in the nacreous layer near the surface region of pearls for different colored pearls. The aragonite sheets hosted in pink pearls were the thinnest. Furthermore, the imbricate structure and morphology were also different on the surface of pearls, and that on the surface of pink pearls was the most inseparable when comparing with white and purple pearls. Hereby, the coloring mechanism of freshwater cultured pearl was attributed to the differences in the thickness of aragonite sheets and surface morphology.