Study on Colour Characteristics and Quantitative Classification of Nanhong Agate Based on HSB Colour Space

In recent years, the price of the Nanhong agate has been rising in jewelry market. As one of the most important factors affecting the quality evaluation of the Nanhong agate, its colour is closely related to the market price. However, the national grading standard of the Nanhong agate has not yet introduced. In order to make the evaluation process of the Nanhong agate colour more objective, scientific and standardized, this paper focuses on the colour characteristics and quantitative classification of the Nanhong agate. The colour images of the samples were obtained by light boxes and cameras. Based on HSB colour space, the colour of the Nanhong agate were standardized and quantified. Origin and SPSS softwares were used to statistically analyze the hue, saturation and brightness (HSB) parameters of the obtained colour data. The results showed that at the 95 % confidence level, the hue (H) of the red Nanhong agate was mainly distributed in a range between3.20°, 17.52°. The saturation (S) and brightness (B) were mainly within 63.82%, 92.69% and 42.97%, 75.37%, respectively. The statistical results were consistent with the visual observed colour characteristics of the Nanhong agate. Combined the characteristics of visual observed colour with the TwoStep cluster analysis, this paper recommends to classify the colour of the red Nanhong agate as follow. The H is divided into three categories, including red (R), orange red (oR), red orange (rO). The S is divided into three levels: S₁, S₂ and S₃. The B is also divided into three levels: B₁, B₂, and B₃. For the saturation (S) in different colour varieties, the "Shizihong""Shuihong" and "Jinhong" (different kinds of red) have greater S than other varieties; the "Binghong" and "Meiguihong" are medium, and the "Bingfen" is the lowest. For brighthess (B), the values of "Bingju" and "Shuihong" are the brightest; the "Shizihong" "Binghong" and "Bingfen" are medium, and the "Jinhong" and "Meiguihong" are the lowest. The colour names of the Nanhong agate in the market are scientifically distinguished and the accurate range of each variety in HSB space is recommended based on the colour distribution characteristics above and the colour grading results.