gems, corundum, color.4, advanced testing, infrared spectroscopy, FTIR

Heat treatment of some ruby and sapphire can be identified by detection of the 3309 cm⁻¹ series in their infrared spectra. The 3309 cm⁻¹ series consists of three peaks at 3309, 3232, and 3185 cm⁻¹, with two additional weak peaks at 3367 and 3295 cm⁻¹. Observation of this series in a ruby or sapphire has significantly different implications than the identification of a single peak at 3309 cm⁻¹. This series represents a specific form of hydrogen incorporation in the corundum structure that is a diagnostic indicator of heat treatment for certain types of ruby and sapphire, particularly ruby from Mozambique and pink sapphire from Madagascar. However, identification of this series can be challenging because other peaks can occur in the same region as the 3309 cm⁻¹ series. This article presents criteria for accurate identification of the three main peaks in the 3309 cm⁻¹ series by measuring their possible peak position ranges, allowing for separation from other possible unrelated peaks in this region. Measurements were made on heated and unheated natural and laboratory-grown blue sapphire and ruby from various origins. With increasing iron and/or chromium content, the positions of these three main peaks shifted narrowly, about 1.7 to 3.6 cm⁻¹, from minimum positions at 3308.9, 3231.2, and 3183.7 cm⁻¹. A positive linear relationship was observed between the positions of three main peaks in the 3309 cm⁻¹ series and concentrations of iron for blue sapphire or the sum of iron and chromium for ruby. These relationships are useful to determine whether peaks in this region actually represent the 3309 cm⁻¹ series or some other hydrogen-related species in the corundum structure. Additionally, peak widths in the 3309 cm⁻¹ series also broaden with increasing iron or the sum of iron and chromium concentrations.