corundum, Burma.12, color.4, inclusions.5, gems, infrared, IR, spectroscopy, ruby, Mong Hsu, heat treatment

The infrared absorption of the rubies from Mong Hsu was analyzed in order to classify the absorption features recorded. In the mid-infrared region, it was possible to distinguish two separate types of spectra. The first type was recorded in non-heat-treated ruby samples in the region from 1900 to 3400 cm⁻¹, identifying inclusions of diaspore. The second type was recorded in heat-treated ruby samples in the region from 3100 to 3600 cm⁻¹, indicating structurally bonded OH groups. Detailed for the first time in corundum, the building of OH groups is accomplished as a direct result of the dehydration of diaspore inclusions, demonstrated through heat treatment experiments.