This content was first published on IGR – Italian Gemological Review no. 21 in 2025. The information provided here is therefore current as of the original publication date.
A recent study titled “New Insights into the Factors Influencing the UV Fluorescence and Colour of Red Spinel from Myanmar” by Wenxi Zhang, Tao Chen, Xiuhong Liao, Xing Xu, Lili Lou, Xuejun Yan, and Jun Yan has provided new perspectives on the factors influencing ultraviolet fluorescence and the characteristic color of red spinel.
The research, published on The Journal of Gemmology, 39(6), 2025, focused on natural spinels ranging in color from vivid pinkish-red to dark red, all with low iron (Fe) content — traditionally considered a fluorescence quencher.
Spectroscopic emission analysis revealed that samples exhibiting strong to medium-strong fluorescence showed a peak at 686 nm, while those with very weak to inert fluorescence displayed a peak at 705 nm. A significant observation was the much higher chromium (Cr) concentration in weakly or non-fluorescent samples (ranging from 25,431 to 29,354 µg/g) compared to the strongly to moderately fluorescent ones (Cr ranging from 4,036 to 8,832 µg/g).
This finding suggests that excessive Cr not only occupies the octahedral sites in the spinel structure but also the tetrahedral ones, leading to structural disorder and acting as a fluorescence suppressor.
Contrary to previous assumptions that attributed reduced fluorescence primarily to iron content, this study found no clear correlation between FeO levels and UV fluorescence intensity in the analyzed samples.
Since sunlight contains a substantial visible light component, spinels with an appropriate amount of Cr exhibit a vivid neon-red color, intensified by the red fluorescence that overlaps with the body color. Conversely, samples with excessive Cr tend to appear darker red and exhibit weak or no fluorescence.
This study highlights chromium concentration as a key factor in determining the intensity of red fluorescence in spinel and the manifestation of neon color under sunlight. Excess Cr acts as a fluorescence inhibitor, likely due to its occupation of tetrahedral sites within the crystal structure. On the other hand, an optimal Cr content allows for strong red fluorescence, enhanced by visible light excitation in sunlight, contributing to the gem’s vivid neon-red appearance.
Gem news published on IGR – Italian Gemological Review #21 – Autumn 2025.
