Spectral Characteristic and Colour Genesis of A Kind of Pink Synthetic Opal

In this paper, the gemmological characteristics, spectroscopic characteristics and colour genesis of a kind of pink synthetic opal were investigated applying ultra-depth microscope, Fourier transform infrared spectroscopy (FTIR), microscopic laser Raman spectroscopy (Raman), ultraviolet-visible absorption spectroscopy (UV-Vis), laser induced breakdown spectroscopy(LIBS), and X-ray fluorescence energy spectroscopy (EDXRF). The pink synthetic opal samples are transparent, show vitreous luster and have no play-of-colour phenomenon. The basic gemmological characteristics were tested with the refractive index of 1.41-1.42 and the density of 2.16-2.20 g/cm³. The spherical and ellipsoidal micro-nano scale solid inclusions with different shades of red were observed under microscopic inspection. The synthetic pink opal samples showed both opal-A and opal-CT characteristics and were closer to opal-A opal by FTIR and Raman. Besides, the FTIR transmission spectra showed a series of peaks at 7 224, 4 523, 3 674, 2 662 cm^-1 and 2 266 cm^-1, consistent with those of the Russian synthetic opal. The content of SiO₂ of the samples is in the range of 97.581%-98.352%; in addition, 0.287%-0.497% of Na₂O, 0.112%-0.326% of MgO, 0.895%-1.507% of Al₂O₃, 0.011%-0.026% of K₂O, 0.003%-0.015% of CaO, and 0.079%-0.244% of Cu₂O were tested. The colour is due to the fine dispersion of mineral particles such as copper oxides (mainly Cu₂O) and the characteristic absorption peak near 570 nm is shown in the UV-Vis spectrum.