Abstract:The coloration mechanism is fundamental to the accurate identification and color optimization of colored gemstones. To gain deeper insights into the coloration mechanism of blue-green tourmaline, this study conducted a series of spectroscopic investigations on blue-green tourmaline mined from Brazil, including routine gemological testing, XRD analysis, FTIR Analysis, UV-Vis spectral analysis, and XPS analysis. The results indicate that the tourmaline is lithium tourmaline, with a relative density of 3.08, No=1.640, and Ne=1.619. A broad, well-symmetric absorption was found at 718 nm in the red region, whereas good transmission was detected in the blue and green regions, which produced the tourmaline’s unique color of bright blue-green. Based on spectral data and analysis of the crystal chemical environment, we infer that the absorption at 718 nm may be caused by Fe2?-Fe3? charge transfer, wherein Fe2? and Fe3? may occupy the Y and Z sites within the crystal lattice, respectively, with mass percentages of 42.1% and 57.9%. Additionally, Mn3? was found, inferred to occupy the Z site within the lattice, but it did not cause selective absorption of visible light. The above research provides in-depth revelation of the chemical states of chromogenic ions in colored tourmaline, offering a reliable basis for the accurate identification and color improvement optimization of tourmaline. For the precise assignment of ion occupancy, future studies could employ more direct characterization techniques such as single-crystal X-ray diffraction refinement of the crystal structure for further verification.