The Quality Evaluation Of Fancy Sapphire Color

In addition to the blue,sapphire’s color ranges from red to purple impacted not only by chemical composition itself but also by external environmental factors greatly（e.g.,observation background）.Gemstones color grading system can be supplemented through quantitative analysis on the above factors.233 fancy sapphire samples as the study object by K-Means method in SPSS to divide the color into 6 series,namely,the red,orange,yellow,green,blue and purple.By Fourier Transform Infrared Spectroscopy（FTIR）,it’s observed that the samples are similar to the corundum Al2O3 standard IR spectra with the absorption peaks near the 621,505 and 463cm^-1 as well as the 3309cm^-1 and 3216cm^-1 absorption peaks related to Fe,Ti,V,etc.The 6 series are found to have distinctive peak intensities through Ultraviolet Visible Spectrophotometer（UV-VIS）,which provides clue to their coloring element content.LA-ICP-MS shows that almost all color series samples come with four elements,to be specific,Mg,Ti,V and Fe with Fe ranks higher.The degree of correlation between coloring elements of different color series and lightness,chroma and hue are different,of which the color are mostly related to Fe and Cr with Cr’s weakening effect on the lightness but promoting the chroma of red sapphires.On the basis of observation backgrounds study,the impact degree on their lightness,chroma and hue angle by the different non-color background can be sorted orderly as the lightness,the chroma and the hue angle.The lightness and chroma increased as the non-color background lightness raises,of which the samples with higher lightness and chroma are largely influenced by the non-color background.Upon the color grading of the yellow sapphires with the widest range of the chroma and lightness,it can be classified into 7 grades.According to the results of LA-ICP-MS,it is found that the Mg and Ti content of high qualiy yellow sapphires is moderate at 53.01-63.49ppm and 73.86-92.80ppm respectively while the Fe is higher at 11542.27ppm.The 470 groups of color data of yellow sapphire samples under different non-color backgrounds can be classified into 9 grades and the quality decreases progressively from B1-B9.The non-color background lightness with the best visual color of yellow sapphires can be clearly quantified based on the 7 color levels.Therefore,N6.5-N8.5,that is,the gray background with lightness of L^*=65-85is recommended for high lightness and low chroma sapphire in daily display;while for the other yellow sapphire,it’s suggested to use the standard white background or background of slightly lower lightness with N7.5-N9.5 of Munsell Neutral Value Scales,namely,the gray background with the lightness of L*=76-95.