Abstract: In 2015, the characteristic quartz jade-purple-green agate, was discovered in the territory of Luonan county, Shaanxi Province, whose typical colour is red, green, and purple.The existing literature mainly focused on the gemmological characteristics and colour genesis of the "red core and green belt" type of purple-green agates^1-3. However, there are still fewer studies on the gemmological characteristics and formation conditions of the purple-green agate with a multicoloured band of orange-white-green. There are still fewer studies on the gemmological characteristics and formation conditions of purple-green agate.In this study, a total of 16 representative samples from the mining areas of Mahuagou, Shibapan, Xuegou, and Zhujiawa in Luonan county, Shaanxi Province, were selected. The gemmological characteristics, mineral compositions, coloration genesis of the ring-banding of the purple-green agate samples were studied using conventional gemmological tests, polarizing microscope, Raman spectrometer, infrared spectrometer, ultraviolet spectrometer, and scanning electron microscope.The main mineral composition of purple-green agate (Fig. 1) is quartz with the particle size about 0.01 mm, and the quartz content is more than 90 %, as well as a small amount of clay minerals, dark metallic minerals and carbonate minerals. Among them, the red impurity minerals are distributed in the form of fine dots or dipping, with fine particle size (1-20 μm); and the green impurity minerals are distributed in the form of flakes and fibres. The results of infrared transmission spectroscopy (potassium bromide tabletting method) showed that there are different degrees of molecular water in the void of the agate samples' microstructure. The red, purple, and colourless agate samples have basically no characteristic peak of structural water at 3 628 cm^-1; the green samples show a significant absorption peak at 3 628 cm^-1, and it is speculated that the structural water contained in the water-bearing minerals causes absorption here. Raman spectroscopy tests revealed a very small fraction of α-quartz spectra with a weak plagioclase signal observed at 508 cm^-1, which is different from the existing literature that concludes that plagioclase is not present in purple-green agates. In the samples with the typical orange-red-white-green ring-banding structure, the main impurity mineral in the orange central area is hematite, and there are also a large amount of dolomite with hematite attached to the surface, showing the orange-red appearance, which contribute to the orange colouration of the central area of the ring-banding; there are a large amount of anatase with a grain size of 0.01-0.02 mm in the colourless ring-banding from the centre area to the edge area.The main impurity minerals in the green edge area are chlorite, sericite, and a small amount of sodium-hornblende is also observed in some samples; the yellow area of sample QZ-1 is mainly composed of dolomite. The UV-Vis spectra of the red and purplish red samples show characteristic peaks of hematite⁴ and a wide range of troughs of iron ions around 740 nm⁵. According to the first-order derivative spectrum peaks show that the more the hematite content is, the more the peak position is shifted to higher wavelengths, from 574 nm to 584 nm (Fig. 2). The first-order derivative spectrum of the yellow area of the UV-Vis spectrum of sample QZ-1 shows the main peak at 530 nm with a larger intensity, accompanied by a secondary peak at 432 nm.The second-order derivative processed spectrum shows the characteristic valley 447 nm of goethite, with a high intensity.Based on the above results, it can be confirmed that the main component of purple-green agate is quartz, and the red, green and yellow area are coloured by impurity minerals.The main red colouring mineral is hematite, and the depth of red is positively correlated with the content of hematite; the chlorite and sericite are the main green colouring minerals, and the yellow area is coloured by goethite. Combined with the occurrence state of the chromogenic minerals, it is speculated that the formation environment of purple-green agate is complex, and chlorite belongs to the metamorphic genesis.The chlorite/illite ratio is low at low temperature (230-260 ℃), and the ratio increases at high temperature (330-350 ℃) According to this chlorite/illite ratio, the temperature range of chlorite formation can be judged⁶. The presence of anatase indicates that the crystallization temperature range of red and green transition ring zone may be 470-450 ℃⁷. The orange-red central area is the iron material carried by the magmatic hydrothermal fluids during the same period, and the later slow cooling deposition stayed in the flint in the form of impurity inclusions along with the siliceous quartz crystals.The microstructure exhibits that a large number of red iron compounds are distributed in the interstices surrounding the quartz grains and wrapped around the carbonate collodion in the form of iron oxides or hydroxides⁸.