When microscopic identification is not feasible, FTIR spectra at high resolution (0.5 cm–1) can distinguish natural and synthetic amethyst. The 3595 cm–1 band is characteristic of natural amethyst and has a full width at half maximum (FWHM) of 3.3 ± 0.6 cm–1. In synthetic amethyst, this band is either absent or (very rarely) about twice as broad. Exceptions to this criterion include natural amethyst with pronounced near-colorless zones, which do not always display this band, and natural specimens that either have an intense, unusually broad 3595 cm–1 band or show total absorption in the X-OH region (3800–3000 cm–1) of the spectrum.
