Colorimetric Study And Quality Evaluation Of Nanhong Agate In Baoshan,Yunnan

Baoshan Nanhong agate is a kind of natural red agate with delicate texture,oil and bright color,which is very popular with consumers.Based on previous studies,this paper analyzed the mineralogical characteristics and Colour-Forming causes of Baoshan Nanhong agate by using conventional Gemological instruments,DiamondView^TM,X-ray fluorescence spectrometer,infrared spectrometer,micro-Raman spectrometer,ultraviolet-visible spectrophotometer and X-rite SP62colour analyzer,quantified the colour parameters of Nanhong agate,summarized the colour rules and tried to classify the colours.By changing the light source and illumination,we can find the most suitable light source and illumination for Baoshan Nanhong agate display and quality evaluation.The conventional gemological characteristics of Baoshan Nanhong agate are similar to those of common agate.The difference is that there are a large number of red dot-like particles with different sizes,which are dot-like,elliptical,long strip-like and aggregate into strip-like,mass-like and irregular distribution.Under DiamondView^TM,fluorescence is generally inert,with a few yellowish-green fluorescence,blue-white fluorescence at bands,reticulate blue-white and red fluorescence at some samples,and red fluorescence at some cracks.X-ray fluorescence spectroscopy showed that the main chemical composition of Baoshan Nanhong agate was SiO₂,accounting for more than 98%,which contained a small amount of S,Fe,Ca,K,Cu,Sc,Cr,Mn and other trace elements.The content of Fe₂O₃ was 0.009-0.055%.Infrared spectroscopy showed that the main mineral was?-quartz,and its degree of crystallization changed from phanerocrystalline to cryptocrystalline,more to phanerocrystalline.Micro-Raman spectrometer showed that the main mineral is?-quartz,which contains a small amount of plagiosilite and calcite,and the chromogenic component Fe2O3 is hematite.Ultraviolet-visible absorption spectroscopy showed that there is a strong and wide absorption band in the purple-blue-green region at 300nm-550nm,and a weak absorption band in the red region at 620nm-660nm.It is inferred that the main chromogenic mineral ion of the Baoshan Nanhong agate is Fe³⁺.By quantifying the color parameters,it is concluded that the main color of Nanhong agate in Baoshan is orange yellow-orange red-red,with medium brightness and medium to low saturation.Through K-Means cluster,its color can be divided into five categories:Pink white?L^*?[48.95,50.11],a^*?[5.64,5.86],b^*?[1.82,3.83]?;Red?L^*?[44.45,46.75],a^*?[27.21,33.60],b^*?[24.43,32.81]?;Brownish red?L^*?[34.66,46.40],a^*?[10.66,19.28],b^*?[8.01,14.87]?;Orange red?L^*?[37.93,45.91],a^*?[20.17,27.07],b^*?[14.37,23.40]?;Orange yellow?L^*?[49.88,53.02],a^*?[14.51,15.65],b^*?[29.42,32.81]?.By comparing and analyzing the parameters of Baoshan Nanhong agate samples under D65,A and CWF light sources,it is determined that D65 light source is the most suitable light source for quality evaluation and display of Baoshan Nanhong agate.2000-2500lx illumination is the most suitable illumination for the display light source of Baoshan Nanhong agate.