Study on Colour and Identification Characteristics of Natural and Dyed Fire Opal in the Market

Fire opal as a rare opal variety in the market has been widely loved. The cause and identification of the colour of fire opal has always been a difficult problem in testing. As the market demand for dyed fire opal identification becomes more and more urgent, it has a certain degree of significance in research and application.This article selected the common natural and dyed fire opals in the market as the research objects. Conventional gemmological tests, microscopic observations, Fourier infrared spectrometer, DiamondView^TM, photoluminescence spectroscopy, ultraviolet-visible spectroscopy and Raman spectroscopy tests were done to analyze the gemmological characteristics and identification methods of common fire opals from different areas on the market, which may provide clues to explore the method of distinguishing natural body colour fire opal and dyed fire opal by conventional laboratory technique.Research results showed that most natural fire opals are uneven in colour to some extent, while dyed samples not. There are some significant differences in the fluorescence images of natural and dyed fire opal under DiamondView^TM. Infrared and Raman spectroscopy can assist in distinguishing the fire opal origin. Dyed samples from Africa had the dye absorption peaks within the range from 1 200 to 1 600 cm^-1. There are also some difference of peak shape and absorption peak between dyed and natural fire opal in PL test. There was no obvious difference between the dyeing fire opal and natural fire opal in UV-Vis spectra.The full contrast of appearance features and the comprehensive analysis of spectral and luminescent characteristics can be used to distinguish the colour origin of the fire opal in practice.