Metameric effects on peridot by changing background color.

Peridot's yellowish-green color is not only produced by ${{\rm Fe}^{2+}}$Fe2+ but is also influenced by background, observer, illuminance, and color temperature of the light source. In this research, we studied the most effective background for peridot color grading under a 6504K fluorescent lamp. Ninety-five gem-quality peridot samples from China, Pakistan, and Myanmar were studied using electron probe micro-analyzer, spectrophotometer, Munsell neutral gray backgrounds, and standard gold alloy backgrounds. The results provided the color effects of 24K, 22K, 20K, 18K, and 16K gold alloy backgrounds and Munsell neutral gray backgrounds on the peridot's color. It was found that the Munsell N9 background seems to be the most effective background for peridot color grading. Using the Munsell N9 background, peridot colors were graded into three levels from good to bad: Fancy Intense, Fancy Deep, and Fancy by using K-means clustering analysis and Fisher discriminant analysis methods. The peridot's color level reverse phenomenon should be considered when the background varies between Munsell N7 and N9 gray.