Feature Gems & Gemology, Fall 2011, Volume 47, No. 3

Infrared Spectroscopy of Natural vs. Synthetic Amethyst: An Update

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.