Gemmological and Mineralogical Characteristics of Dark-Green, Bluish-White and Pink Seal Stone Materials

In recent years, various types of seal stone materials have emerged in China's jewelry market, and some products being sold as Shoushan stone or Qingtian stone at high prices. In this paper, the gemmological characteristics, mineral components and microstructure of three types of seal stone samples with different colours including: dark-green, bluish-white and pink, were analyzed by conventional gemmological tests, Fourier transform infrared absorption spectrometer, UV-Visible spectrophotometer, Raman spectrometer, X-ray powder diffractometer, inductively-coupled plasma mass spectrometer and scanning electron microscope. The results show that the minerals in the three types of seal stone samples are not directionally distributed in thin sheets, and the gemmological and spectroscopic characteristics are also different due to the various types of minerals they contain. The main mineral components of the dark-green seal stone samples are about 85% phlogopite (the diffraction characteristic peaks are 1.001 9, 0.502 3, 0.335 3, 0.201 3 nm) and 14.1% talc(the diffraction characteristic peaks are 0.932 0, 0.458 1, 0.311 2, 0.291 6, 0.261 4, 0.167 4 nm). The main mineral components of the bluish-white seal stone samples are inclined chlorite(the diffraction characteristic peaks are 1.419 9, 0.710 9, 0.473 6, 0.459 1, 0.354 8, 0.284 1, 0.253 6 nm). The main mineral components of the pink seal stone samples are talc and serpentine(the diffraction characteristic peaks are 0.724 9, 0.360 7, 0.233 9, 0.209 8 nm). The combination of chemical compositions and UV-Visible spectra show that the colour origin of dark-green seal stone is related to Fe, which is a joint action of the ⁶A₁→ ⁴T₂(D) electron transition of the Fe³⁺ d orbit and the charge migration between Fe²⁺-Fe³⁺; the colour origin of bluish-white seal stone is mainly related to Fe²⁺, and the migration of Fe²⁺-Ti ⁴⁺ charge makes a little contribution.