COLOR ORIGIN OF COLOR CHANGE SAPPHIRE

Color change, or the inconstancy effect, refers to significant changes in the perceived color of an object across two or more different lighting conditions, such as daylight and incandescent light. The color change effect was first discovered in alexandrite (Cr-bearing chrysoberyl) and has also been observed in other gemstones, for example, sapphire, diaspore, garnet, etc. In nature, sapphire exhibits an extremely broad range of hues. Only a small number of sapphires can be identified as exhibiting color change, yet the color origin of those sapphires is not fully understood. There are six major chromophores that cause the color in sapphire [1]. Except for V³⁺, other chromophores (Cr³⁺, Fe³⁺, Fe²⁺-Ti⁴⁺, h-Cr³⁺, and h-Fe³⁺) can't generate the color change effect individually. However, due to the low concentration of V³⁺ in natural sapphire, V³⁺ is not a very common chromophore in color change sapphire. Therefore, the interaction of several chromophores must be responsible for causing the color change phenomenon in sapphire. In this research, we simulate the spectra of sapphire with various combinations of different chromophores. The colorimetric data is calculated from those simulated spectra. In order to investigate the possibility of a chromophore combination causing a color change effect, the color difference value between incandescent light and the D65 lighting condition has been computed and compared with the threshold value of color inconstancy [2]. Three categories of color change sapphire are proposed based on our simulated results and trace element data from actual color change stone:

1. Blue to purple sapphires which contributed by (Fe²⁺-Ti⁴⁺ and Cr³⁺)
2. Green to reddish brown sapphires which contributed by (Fe²⁺-Ti⁴⁺ and Fe³⁺)
3. Pink to purplish brown sapphires which contributed by (Fe³⁺ and Cr³⁺)

Reference

1. Dubinsky, Emily V., Jennifer Stone-Sundberg, and John L. Emmett. "A quantitative description of the causes of color in corundum." Gems Gemol 56.1 (2020): 1-27.
2. Shen, Che, and Mark D. Fairchild. "The threshold of color inconstancy." Color and Imaging Conference. Vol. 2021. No. 29. Society for Imaging Science and Technology, 2021.