Explanation of the alexandrite effect of zultanite: From the view of colorimetry and chemical analysis.

Zultanite is the commercial name of a gem‐quality transparent diaspore that exhibits an obvious color change when exposed to different light sources. It usually appears green under daylight and red under incandescent light. This optical feature makes zultanite valuable, and the study of the spectral characteristics, as well as chemical composition, of zultanite can increase our understanding of the associated color change mechanism. Spectral transmittance curves for zultanite exposed to nonpolarized light emitted by CIE standard illuminants D65 and A show that the color change phenomenon is related to the optical characteristics of light sources. The colorimetric parameters in the CIE 1976 L*a*b* uniform color space system illustrate that the color change exhibited by zultanite is an example of the alexandrite effect, which can be quantitatively described by the significant change of a* and h° parameters. These findings and the results of chemical element analysis by X‐ray fluorescence and X‐ray photoelectron spectroscopy imply that electron transitions between different energy levels involving the 3d orbitals of the trace elements Fe3+ and Cr3+ in zultanite give rise to the relatively high transmittance observed in the green region (around 500 nm) and the orange‐red region (600‐780 nm) of visible light. Thus, the difference in the spectral power distributions of different ambient light sources greatly influence the color of zultanite, making zultanite show green under daylight which is richer in the green light and turns red under incandescent light in which the orange‐red light dominates. Summary: The main contributions of this article can be summarized as follows: The colorimetric parameters of zultanite illustrate that its color change phenomenon is an example of the alexandrite effect.The trace elements Fe3+ and Cr3+ are the essential factors that cause the alexandrite effect, which give rise to the relatively high transmittance observed in the green and orange‐red regions of visible light.This article will inspire deeper research on the optical properties and application of hydroxides. [ABSTRACT FROM AUTHOR]