Gemmological Characteristic of Nanhong Agate and Formation of Red StriDe

The authors used X-ray powder diffraction (XRD), scanning electron microscope (SEM), infrared spectrometer, laser Raman spectrometer, laser induced breakdown spectroscopy (LIBS) and other modern advanced analytical instruments to study the mineral components of Nanhong agate in order to understand the influence factors of colour and to explain the formation of red stripes. The refractive index of Nanhong agate is about 1.54 to 1.55, and relative density is 2.642. These agates show no reaction under the ultraviolet lamps and show cryptocrystalline structure, conchoidal fracture and glassy luster. The major mineral component of Nanhong agate is quartz, with high content of Si, followed by Fe and trace elements such as K, Ca, Al, Mg and Cl. Furthermore, the intensity of red colour is directly proportion to the Fe contents. The laser Raman spectroscopy results show that the red parts show the presence of hematite. The reasons for the formation of the red stripes in Nanhong agate can be summarized as follows:the magmatic fluid in the cavity causes periodic crystallization of the agate. Fe is in the form of independent iron mineral (hematite), which makes the agate display red colour. And the higher iron oxide content, the deeper the red colour. In terms of pattern, when the crystallization rate is proper, the injected fluid can evenly distributed to form the parallel stripes. However, when the crystallization rate is too fast, the fluid does not have sufficient time to even distribution and thus it leads to the formation of messy lines which may look like flames.