Gemmological Characteristic of Clay Mineral Jade ("Changhua Zhushashi")

Changhua, Zhejiang Province is an important place of seal stone production in China. There are various kinds of colourful seal stones produced in Changhua, Zhejiang Province. The object of this study is maroon-red "Changhua Zhushashi". Polarizing microscope, Fourier transform infrared spectrometer(FTIR), X-ray powder diffractometer(XRD), laser Raman spectroscopy(LRM), and scanning electron microscopy(SEM) were applied to study the gemmological characteristics of the samples. The results showed that the maroon-red "Changhua Zhushashi" is clay mineral jade with cryptocrystalline structure, and is composed of dickite, kaolinite and hemitate. Crystals of dickite and kaolinite are mostly irregular subhedral to anhedral sheet, with poor orientation, and particle size ranging from a few microns to a dozen microns. According to the crystal morphology, hematite is characterized by two crystalline forms: The first hematite type is a regular hexagonal crystal size of 30-400 μm, which can form optical phenomena on the surface of polished finished products; The second is aggregation of tiny particles about 0.5 μm in size, and its colour is maroon. It is speculated that these two crystalline forms of hematites are formed in the stage of hydrothermal alteration and contact metasomatism respectively. Furthermore, maroon-red "Changhua Zhushashi" contains pyrite and quartz.