Rapid screening of a photochromic Padparadscha-like sapphire using absorption spectroscopy

The Padparadscha sapphire is attracting increasing interest due to its unique color combination of pink and orange. However, some Padparadscha sapphires exhibit an unstable orange color component; the stone turns from pink to orangey pink after longwave ultraviolet (UV) radiation, and the orange color fades out under a room light or halogen light source. This is of interest and concern since accuracy and consistency are important in colored stone identification services. This study investigates the UV visible (UV-Vis) absorption spectra of photochromic Padparadscha sapphires under illumination of different wavelengths between 250 and 600 nm. Our results show that shortwave UV induces the orange color more effectively than longwave UV, and Vis light near 450 nm reduces the orange color more effectively than a halogen light source. Based on these findings, we developed a prototype UV-Vis absorption spectroscopy device with two fiber-coupled light-emitting diodes at 265 and 455 nm. It can perform rapid screening of photochromic Padparadscha sapphires and has faster color recovery compared to current techniques.