Identification Characteristics of Natural and Synthetic Amethyst by Infrared and Polarized Raman Spectroscopy

Abstract

BACKGROUND The identification of natural and synthetic amethyst is still a challenge for gemological laboratories either at home or abroad. Previous studies focus on twinning, color band, inclusions and the infrared spectrum. Controversy still exists on whether the 3595cm-1 and 3543cm-1 absorptions are indicative or conclusive clues when employing infrared spectroscopy to separate natural amethyst from synthetics. OBJECTIVES To characterize the features of natural and synthetic amethyst. METHODS Typical natural and synthetic amethyst samples were collected. The limitations of infrared spectroscopy to identify natural and synthetic amethyst were studied. The polarized Raman spectroscopy was applied to the identification of amethyst genesis. RESULTS The results showed that there were some limitations in the origin determination of amethyst by using the infrared absorption peaks of 3595cm-1 and 3543cm-1 as criteria. These peaks were indicative, but cannot be used as a decisive basis. The polarized Raman spectrum was complementary to infrared spectroscopy. The polarized Raman spectrum (the direction of both incident laser and Raman signal set as horizon-HH) peak of 400cm-1 were observed in all the natural amethyst, but this peak was absent in the synthetics. The polarized Raman spectrum (direction:HH) peaks of 795cm-1 and 448cm-1 appeared in all the synthetic amethyst, but were not detected in natural amethyst. CONCLUSIONS The difference of polarized Raman spectra may be related to different degree of crystal lattice deformation in natural and synthetic amethyst. The polarized Raman peaks of 400cm-1, 448cm-1 and 795cm-1 can be used as a new criterion for source determination of amethyst.

Keywords

• natural amethyst
• synthetic amethyst
• infrared spectroscopy
• raman spectroscopy
• genesis identification