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.
Raspberry-red grossular, found exclusively in the Sierra de Cruces in Coahuila, Mexico, has been the subject of a scientific study titled “Chromogenic Mechanism and Formation of Zonal Genesis of Raspberry-Red Grossular from the Sierra de Cruces Range, Mexico”, conducted by a research team led by Siyuan Wu, Siyi Zhao, Yi Zhao, and Chenxi Zhang and published on Minerals (2025). This gem stands out for its distinctive color zoning, characterized by an outer pink rim, a black intermediate zone (mantle), and a white core.
In addition to standard gemological testing such as refractive index and specific gravity measurements, advanced spectroscopic techniques were employed. Chemical analyses proved crucial in correlating composition with the observed color zoning. These data revealed a clear relationship between manganese (Mn) content and the pink coloration of the rim, with MnO concentrations significantly higher in this area (0.15 wt% – 1.72 wt%). Simultaneously, the black mantle was found to be rich in iron (Fe), with notably higher FeO content (3.11 wt% – 5.09 wt%). It is suggested that the iron in the mantle is primarily present as Fe3+, partially substituting for Al3+ in the garnet structure, resulting in the darker coloration.
The detailed study conducted by Wu and colleagues elucidates the mechanisms responsible for the unique coloration of raspberry-red grossular, identifying the key role of manganese in producing the pink hues and of iron in generating the black tones. Furthermore, the geochemical analysis allowed researchers to reconstruct the varying environmental conditions that led to the formation of the concentric zoning. This research provides a significant contribution to the understanding of zoned gemstones and the geological processes underlying their formation.
For more details, you can read the full article at the following link: https://www.mdpi.com/2075-163X/15/2/138
Gem news published on IGR – Italian Gemological Review #21 – Autumn 2025.
