Mineral Component and Colour Genesis of the Circular Bands of Purple-Green Agate from Shaanxi Province, China

Purple-green agate is a kind of characteristic quartz jade within the territory of Luonan County, Shaanxi Province, with the colours of red, purple, green, and white as the main colours, with occasional yellow-toned and orange-toned outputs, and the colours are distributed in the form of bands or clusters. The gemmological characteristics and the causes of the ring-band coloration of the purple-green agate from Luonan County, Shaanxi Province, were studied and analyzed by conventional gemmological instruments, infrared spectrometer, ultraviolet spectrometer, Raman spectrometer, scanning electron microscope and other testing methods. The study shows that the purple-green agate has a high degree of crystallization, and the main constituent mineral is quartz, which is uniformly distributed and contains a small amount of chlorite, hematite, calcite, dolomite, sericite, anatase and other impurity minerals. Among them, hematite is the main red chromogenic minerals, and the depth of the red colour and the content of hematite in the sample is positively correlated, chlorite is the main green chromogenic minerals, often accompanied by sericite, and the main components for the yellow part is dolomite. The colour of purple-green agate shows the characteristics of the distribution of the ring zone such as red center and green edge, and the presence of the characteristic impurity anatase in the transition zone from green to red indicates the difference in the geological environment of the formation of the ring zone of the samples.