Gemmological and Mineralogical Characteristics of Green Muscovite Seal Stone ("Ya'an Green")

Generally, blue or green seal stones are rare in nature. Green muscovite Ya'an seal stone(Ya'an green)with its vivid green colour has become an important variety of seal stones in recent years. So far, there are few reports about this kind of seal stone, and it is easy to be confused with other green seal stones. Therefore, an in-depth study on this seal stone variety was conducted, which could provide data reference for the identification of this kind of green seal stones. Samples of "green muscovite Ya'an seal stone" were investigated by X-ray diffractometer, Raman spectroscopy, electronic microprobe analysis, environmental scanning electron microscope and ultraviolet-visible spectroscopy to obtain its mineral components, micromorphological characteristics, chemical compositions and cause of colour formation. XRD, Raman and electronic microprobe results show that the green parts of the seal stone samples are muscovite, and the yellow impurity minerals are mainly dolomite and pyrite with a small amount of quartz, apatite and rutile. Environmental scanning electron microscope results show that the shape of muscovite is false hexagonal platelet arranged in a disorderly way in three dimensional space. The idiomorphic pyrite, and massive dolomite with perfect cleavage can be seen. Through UV-Vis analysis, it is speculated that the vivid green colour of the "Ya'an green seal stone" is mainly caused by Cr element in muscovite.