gems, corundum, color.4

The 3161 cm⁻¹ infrared (IR) band can be observed in variously colored natural corundum, most commonly in yellow sapphire. The presence of this feature is used frequently as supporting evidence to conclude that a sapphire has not been heat treated (C. M. Breeding and N. J. Ahline, “Infrared spectroscopy and its use in gemology,” Winter 2024 G&G, pp. 474–492). This 3161 cm⁻¹ band is often present with other less intense side bands known as the “3161 cm⁻¹ series” and includes features at ~3355, 3242, 3100 (shoulder), 2460, and 2420 cm⁻¹ (C. P. Smith and C. van der Bogert, “Infrared spectra of gem corundum,” Fall 2006 G&G, pp. 92–93). The exact origin of the 3161 cm⁻¹ band is not well understood, although it is thought to be related to stretching vibrations of hydroxyl groups caused by interstitial H⁺ ions present as charge compensation for Fe²⁺ substituting for Al³⁺ in the corundum structure (M. C. Jollands et al., “Vibrational properties of OH groups associated with divalent cations in corundum (α-Al₂O₃),” European Journal of Mineralogy, Vol. 35, No. 5, 2023, pp. 873–890).