Reversible photochromic effect in natural gemstone sapphires

Photochromic sapphires show an optically controllable orange coloration under ambient temperatures, which strongly affects the color appearance and market value of gemstone sapphires. An in situ absorption spectroscopy technique using a tunable excitation light source has been developed to investigate the wavelength- and time-dependence of sapphire's photochromism. Excitations of ≤370 nm and ≥410 nm introduce and remove orange coloration, respectively, whereas there is a stable absorption band at 470 nm. Both the color enhancement and diminishing rates are proportional to the excitation intensity; hence, strong illumination can significantly accelerate the photochromic effect. Finally, the origin of the color center can be explained by a combination of the differential absorption and the opposite trends between the orange coloration and Cr³⁺ emission, indicating that the origin of this photochromic effect is related to a magnesium-induced trapped hole and chromium. The results can be used to minimize the photochromic effect and improve the reliability of color evaluation for valuable gemstones.