Classification and Microscopic Characteristic of "Jinmai" Agate from Heishan, Xinjiang

As a special variety of quartz jade, "Jinmai" agate, that has tendon patterns on the surface, is deeply loved by consumers. In recent years, the "Jinmai" agate produced near Heishan, Bachu County, Kashgar Prefecture, Xinjiang Uygur Autonomous Region, has become one of the most popular varieties of "Jinmai" agate due to its varied colours and various shapes.More than 150 samples of "Jinmai" agate from Heishan, Xinjiang, were studied and classified by conventional gemmological tests, and its structural characteristics were observed by a polarized light microscope, and the microstructure causes of tendon-pattern on the surface and other special surface structures were analyzed. The results show that the colour of "Jinmai" agate from Heishan, mainly includes five categories: gray-green to dark green series, orange-red to dark red series, orange-yellow to khaki series, milky-yellow to yellow series, and black to gray series. There are "tendons", thick "tendons", internal "tendons", external "tendons", pure "tendons" and "fish roe" structures, and the classification is based on the thickness and distribution density of the "tendons", the position of the "tendons" in the agate, and some special structures, such as annular protrusions and globular protrusions. The microstructural features of "Jinmai" agate from Heishan, Xinjiang are mainly cryptocrystalline structure and cryptocrystalline fiber structure, among which the external structural features of "tendons-pattern" and "fish roe" correspond to the internal cryptocrystalline fiber structure: fiber parallel structure and fibrous radiating structure; while the matrix (the "flesh" substance without special structural features) is partly composed of a cryptocrystalline structure.Raman spectra showed that the main mineral component of "Jinmai" agate from Heishan, Xinjiang is α-quartz, the secondary mineral is moganite and the red globular impurity mineral in some samples is hematite.