Gemmological Characteristic of Dyed Agate Similar to Turquoise

Abstract

Recently, a kind of emerald green gemstone named "Turquoise agate" appeared in the market which is similar to the high-quality turquoise. In this paper, the gemmological characteristics, mineral components, spectral characteristics and colour genesis of the green agate were determined by Fourier transform infrared spectrometer, Raman spectrometer, X-ray fluorescence spectrometer and UV-visible spectrophotometer. The results showed that the green colour is unevenly distributed on the surface of the sample which was caused by the dye diffusion from the outside to the inside. X-ray fluorescence spectra revealed that the main chemical composition of the sample is Si, with a small amount of Al, Mg, Na and trace elements such as Mn, Fe, Cr. There is a higher concentration of Cr in the green part of the sample. Characteristic peaks of the sample in FTIR were matched with the quartz. Meanwhile, the absorption bands caused by organic matter were observed. Raman spectra of the samples showed the characteristic peaks of alpha quartz and oblique silica. UV-Vis spectra showed that the green agate sample has the characteristic absorption band of Cr6+ near 267 nm and other bands caused by the d-d transition of Cr3+ ion in the octahedral field. In conclusion, the sample is not a natural agate as claimed by the merchants, but an enhanced gemstone. Its green colour is caused by dyeing with Cr-containing stain. According to the GB/T 16552-2017, it should be named as agate.