This study, conducted a spectroscopic analysis of 10 gem-quality blue tourmaline samples from Minas Gerais, Brazil, focused on detailed variations in their infrared, Raman, and UV-VIS spectra. Conventional gemological tests, electron-probe microanalysis, infrared spectroscopy (mid- and near-infrared), Raman spectroscopy, and UV-visible spectroscopy were used to systematically analyze the chemical composition and spectral characteristics of the samples. The infrared spectra revealed vibrations of [YO₆], [TO₄], [BO₃], [OH], and H₂O groups, indicating different bonding profiles, with the [OH] vibrational frequency showing a direct correlation with FeO and MnO content. The Raman spectra primarily reflected the stretching vibrations of metal–oxygen bonds and hydroxyl groups, indicating the complexity of the local environment in the crystal structure. The UV-VIS spectra showed that the broad absorption band around 725 nm was due to intermetallic charge transfer between Fe²⁺ and Fe³⁺. This work provides new insights into the local bonding environment within the crystal structure by providing precise spectral data of natural blue tourmaline, and a more accurate classification and evaluation of blue tourmaline through fine spectral change characteristics related to crystal chemistry has important implications for both academic research and the gemstone industry.

Gem-quality blue octahedral crystalline gahnite was produced in Nigeria. This paper investigated gemological and spectroscopic characteristics by basic gemological experiments, electron probes, infrared reflectance spectroscopy, laser Raman spectroscopy, photoluminescence spectroscopy, and ultraviolet-visible spectroscopy. The results show that the refractive index (RI) of Nigerian gahnite is 1.7921.794, and the specific gravity is 4.454.66, with no fluorescence. The main chemical composition is ZnAl₂O₄, accounting for 93.57, and the rest is mainly FeAl₂O₄, which also contains Na, Mg, Co, Mn, Cr, Cu, Si, K, and Ca elements. The infrared spectra showed midinfrared absorption bands near 510cm⁻¹, 559cm⁻¹, and 664cm⁻¹ in the fingerprint region, corresponding to the Zn-O stretching vibration, bending vibration, and Al-O bending vibration, respectively. The Raman spectra showed three of the five Raman active modes of the spinel group, with characteristic Raman absorption peaks located at 418cm⁻¹, 508cm⁻¹, and 660cm⁻¹, corresponding to E_g, T_2g(2), and T_2g(3) modes, respectively, and the comparison revealed a higher degree of Zn and Al ordering in this paper for gahnite. The photoluminescence spectra show the common Cr³⁺-activated fluorescence splitting peaks of natural spinel, of which the 686nm (R-line) fluorescence peak is obvious and sharp. The UV-vis absorption spectra located at 444nm and 489nm are the most obvious, which are caused by the d-d electron leap of ^TFe²⁺ (⁵E⁵T₂), and the blue-gray tones of the samples are mainly caused by the spin-forbidden electronic transitions in ^TFe²⁺ and ^MFe^2+MFe³⁺; the weak absorption peak at 609nm was determined to be associated with Co²⁺ by derivative spectra.