Genesis of the Body Color of Brazilian Gem-Quality Yellow-Green Opal

Abstract

The chromaticity characterization and influencing factors of the body color of 28 yellow-green opals were studied through Fourier transform infrared spectrum (FTIR), energy-dispersive X-ray fluorescence spectroscopy (ED-XRF), ultraviolet-visible spectroscopy (UV-vis) and X-Rite SP62 spectrophotometer. It was found that the hue h° and the iron content of the yellow-green opal were highly negatively correlated, the chroma C* and the sum of nickel and iron content were significantly positively correlated, and the lightness L* was significantly negatively correlated with the chromium content. The body color of light green opal is mainly caused by nickel. The iron content will make the color yellow, and the chromium content will darken the green. With the increase of Fe content, the lowest absorption value of UV-vis absorption at 500–650 nm moves towards the long wave direction, and the hue h° decreases. With the increase of the total content of Ni and Fe, the 650 nm absorption peak area and the chroma C* increase. Under CIE D65 standard light source and N9.5 Munsell neutral background, using K-Means clustering method, 28 yellow-green opal body colors were divided into three grades: (1) Fancy Intense, (2) Fancy Deep and (3) Fancy.