Gemmological Characteristic of Nanhong Agate from Lijiang, Yunnan Province

Nanhong agate from Lijiang, Yunnan Province is produced from the Jinsha River and the basin of Yongsheng in Lijiang, Yunnan Province. It is produced as a seed material with few cracks and high transparency. In this paper, optical microscopy, infrared spectrometer, Raman spectrometer, X-ray diffraction(XRD) and electron probe analysis(EPMA) were used to find the characteristic of structural, mineral composition, chemical composition and the appearance of colouring minerals of Nanhong agate from Lijiang, Yunnan Province. The structural of Nanhong agate from the surrounding rock to the centerare mainly from fine-quartz, short-fibrous, long-fibrous, and sometimes euhedral crystalline quartz is often formed in the middle.The mineral composition is mainly quartz (mass fraction close to 98%) and contains a small amount of water, hematite and other secondary minerals. The form of water in Nanhong agate is mainly divided into two kinds. One kind is the molecule of water, and the other one is connected to the silicon hydroxyl groups. 7 100 cm^-1 in the near infrared spectra is -OH stretching vibration, 5 000 cm^-1 is mainly the expansion of water molecules and bending vibration, the vibration of the 4 500 cm^-1 is on behalf of the Si-OH. The splitting degree at 400 cm^-1 and the XRD spacing at 3.335 7? and 4.242 3? indicate the high crystallinity of Nanhong agate in Lijiang. The colour of the area is generally not bright and the transparency is high. The content of other photochromic ions is not found when the electron probe is tested on the substrate. The laser Raman test for its coloring minerals is hematite and does not contain goethite. Although the red-yellow interfacial fine-grained spherical aggregates can be seen under the optical microscope. It was observed that the chromogenic minerals of the orange-red samples were mostly in the range of 5-10 μm and presented different layers of structures, with black iron in the center and red dye in the outer layers. For the darker samples, chromogenic minerals were in the range of 10-40 μm and distributed in a dispersed aggregate form, so it was speculated that the crystallinity also had an impact on the colour. In conclusion, the colour is mainly related to the colour-developing efficiency, and the colour-forming efficiency includes the crystallinity, distribution mode, and particle size of the colour-developing mineral.