Study On Origin, Factors And Grading Of The Color Of Turquoise From China

Turquoise has been deeply loved by people at home and abroad since ancient times.With the increasing popularity of turquoise,people try to use different ways to improve the color of turquoise to meet the market's pursuit of high-quality turquoise,while color is one of the significant factors to evaluate quality and price of turquoise.So it is necessary to further study the color origin and factors of turquoise and establish a color grading standard of turquoise.The gemological and color characteristics of 14 different colors and 4"raindrops"of turquoise are analyzed by Raman microscope,micro-infrared,micro-area X-ray diffraction,EPMA,LA-ICP-MS,micro-ultraviolet-visible spectrometer,UV-visible spectrophotometer.The results showed that the color of turquoise is not only related to Cu and Fe,but also the trace elements of V,U,Ni and some rare earth elements.It is also found that minerals containing V and U can cause the secondary fancy and dark intense green of turquoise.While yellow and maroon turquoises are caused by hematite and goethite.And the colors of turquoise can become much deeper with its appearance of raindrop and vein when mixed with fluorapatite.In addition,the scanning electron microscope analysis revealed that the pores on the surface of turquoise are unevenly distributed and rich in form and size.With the density of turquoise between 2.19 and2.83 g/cm³,their porosity ranges from 0.044%to 2.141%when it was magnified to 40times by SEM.We found that the saturation of turquoise became stronger and the lightness became darker when the pores were filled with water,glue,wax.So the change of color also indicates that the pores of turquoise play a role in its colors.In conclusion,the color of turquoise is a comprehensive manifestation of the selective absorption,scattering and reflection of light by physical and chemical factors such as its chemical composition,mineral composition and structure.The paper put forward new perspectives and understanding about the color origin of turquoise and enriches the study of difference color of turquoise.51955 turquoise samples from Hubei and Anhui were collected based on the big color data.Furthermore,a model of color space was established for turquoise from China.The results showed that in china,the hue of turquoise ranges from 12°to 220°,including blue,bluish green,green,yellow and so on.We also found that the number of samples and the range of brightness and saturation are not consistent with different hue.Combining usual color terminology and the characteristics and number of turquoise,the hue of turquoise can be divided into six categories containing blue,greenish blue,blue green,green,yellow and orange;the brightness can be divided into three levels including B₁,B₂ and B₃;the saturation can be divided into four levels including light,medium,vivid and intense.Instituting the color grading standard and reference cards of turquoise is aimed to apply to the routine rating activities,which is of great value and significance to the market.