The Spectral Characterization of Blue Spinel and Other Blue Gemstones with the Alexandrite Effect

In gemmology, the term "Alexandrite effect" is used to describe colour change phenomenon when a gemstone is observed under different light sources, usually between daylight and incandescent light. The definition of the Alexandrite effect is constantly being broadened with new discovery of gem resource. The traditional definition of the Alexandrite effect attributing the colour change phenomenon to the presence of two maximum transmission regions and a maximum absorption region in the absorption spectra. In this study, 7 blue spinels and 5 blue gemstones (including tanzanite, kyanite, fluorite, and 2 sapphires) showing the Alexandrite effect were investigated. The goal is to explain the cause of blue-to-violet alexandrite effect and the spectral features causing such colour change. In the UV-Vis spectra, all samples showed a maximum absorption peak in the range of 534-610 nm, within the green region to orange region. The traditional explanation of green to red Alexandrite effect required a transmission window in the red region; however, some of our samples did not show this transmission window and the blue-to-violet alexandrite effect was still visible. Therefore, it is incomplete to explain the mechanism of the Alexandrite effect according to their characteristic absorption spectra, a systematic study based on modern colour science and colour perception in human vision is required to elucidate the blue-to-violet alexandrite effect.