Study of Natural and Synthetic Yellow–Orange Sapphires Based on the CIE 1976 L*a*b* Uniform Colour Space

Abstract

The natural and synthetic yellow–orange sapphires are researched in a new angle of gemstone chromaticity. The relationship between spectrum and composition and colour was studied by UV-Visible spectroscopy and Laser Ablation Inductively Coupled Plasma Mass Spectrometer (LA-ICP-MS) testing. The results show that synthetic yellow–orange sapphires for this article are co-coloured by nickel and chromium, and differ from natural yellow–orange sapphires. The lightness L* of synthetic sapphires is mainly influenced by the chromium, and the hue angle h° is influenced by nickel and chromium, in addition to the ratio of chromium to nickel content. In the UV-Visible spectroscopy, the hue angle of natural sapphire is influenced by the wavelength that corresponds to the trough in the blue–violet area, while the absorption band at 555 nm in the ultraviolet-visible spectrum of synthetic sapphire influences the hue angle. The lightness of the sapphire is higher under A than under D65; the chroma decreases with the change of light source from D65 to A for the yellow sapphire and increases for the rest; the hue angle is generally higher under D65 than under A, and as the red hue continues to increase, the hue angle is higher under A than under D65.