The proper use and limitations of IR spectroscopy for identifying natural versus synthetic amethyst of various types have been investigated, focusing on the region 3800–3000 cm-1. The presence of absorption bands at approximately 3680, 3664, and 3630 cm-1 unambiguously proves artificial origin, but only for samples grown in near-neutral NH4F solutions. Conversely, there are no unambiguous diagnostic features in the IR spectra of the more commercially significant synthetic amethyst grown in alkaline K2CO3 solutions. Nevertheless, previous investigators have found potential diagnostic value in absorption bands at approximately 3595 and 3543 cm-1. Although the 3595 cm-1 band is not found in the spectra of synthetic amethyst, it also is frequently absent from those of natural amethyst. The 3543 cm-1 band is found in the vast majority of synthetic amethysts grown in alkaline solutions, but this band also is sometimes present in natural amethyst—so it provides only tentative evidence of synthetic origin. Moreover, the 3543 cm-1 band is absent from some varieties of synthetic amethyst. The unambiguous identification of natural versus synthetic amethyst therefore must be based on a combined examination of the IR spectra, internal growth structures (including twinning), and inclusions.
Table 1: Presence or Absence of the 3543 cm-1 Absorption Band in Synthetic Quartz According to Growth Conditions
Submitted by Vladimir S. Balitsky