The optical absorption coefficient of five different natural garnet crystals has been measured at 300 K over the wavenumber range from 1000 to 30 000 cm-1. Peaks in the absorption spectrum associated with OH, Fe2+, Fe3+, Mn2+ and Cr3+ ions have been identified. Pure Mg3Al2Si3O12 and Ca3Al2Si3O12 appear to have an intrinsic absorption coefficient below 1 cm-1 in the wavenumber range 3000–30 000 cm-1. There are intrinsic multiphonon lattice peaks at wavenumbers of about 1790 and 2600 cm-1 in all five garnets.
GLEN A. SLACK and R. M. CHRENKO, "Optical Absorption of Natural Garnets from 1000 to 30 000 Wavenumbers," J. Opt. Soc. Am. 61, 1325-1329 (1971)