THE CAUSE OF COLOR VARIATION IN WATERMELON TOURMALINE: INSIGHTS FROM INFRARED SPECTROSCOPY

Watermelon tourmaline is known for its dramatic and vivid pink-to-green color transition within single gemmy crystals. To date, there is no consensus within the scientific community regarding the mechanism causing the color variation. Two contrasting mechanisms can account for its cause. One theory advocates that the different colors reflect the changing chemical conditions while the crystals were growing. That is, pink regions in the core of the crystal incorporated pink coloring agents early in their growth, while later, outer regions of the crystal grew while incorporating green coloring agents. A second potential mechanism that accounts for color variation invokes chemical diffusion after crystal growth. That is, the crystal grew with pink coloring agents, and subsequent thermal soaking then resulted in either the progressive loss or gain of contaminants to generate the zoned color sequence. Here, we present results using high-resolution infrared and visible spectroscopy and microprobe analyses to offer insights into this question. Our results on two crystals (collected from Afghanistan and Brazil) show a correlation between observed color and species absorption at 3660 cm^-1. All data collected from pink regions of the crystals show Abs/mm > 2.1 and all data from green regions are <2.1, indicating that the abundance of contaminant species at 3660 correlates with the abundance of the species responsible for absorbance in the visible spectrum. This result is consistent with the notion that partial diffusive loss of contaminants generated the observed dramatic variation in color. However, we additionally observe ~constant values in absorption at 3660 throughout the green regions of the crystal, with no additional diffusive loss toward crystal edges. Our results suggest that crystal color is correlated with a distinguishable contaminant fingerprint incorporated during crystal growth and can reflect changes in environmental factors that prevailed during this time.