Mineralogical and Spectral Characteristics of Purple Boulder Opal from Australia

Purple boulder opal is a relatively new gem variety in the market. At present, the research that related to purple boulder opal is limited to the mineral components and the genesis of its colour. There is no in-depth analysis on the chemical compositions, spectral characteristics and causes of play-of-colour effect. The mineralogical and spectral characteristics of purple boulder opal from Australia are comprehensively studied in this paper. The results showed that the main mineral component of purple boulder opal is amorphous opal, and it contains a small amount of α-cristobalite. The results of EMPA analysis showed that the main component of opal matrix is SiO2, which is more than 96%. The UV-Vis spectrum showed that there are absorption peaks of strong absorption intensity at 407 nm and 480 nm, and totally absorbed above 522 nm. Based on the results of LA-ICP-MS and UV-Vis absorption spectra, the saturation of the purple colour of purple boulder opal is positively correlated with Fe content. SEM results showed that the diameter of SiO2 spheres in the play-of-colour effect area of the purple boulder opal is about 182-240 nm, and the octahedral space around the sphere is about 63-150 nm. Only violet light with wavelength of 400-450 nm can pass through the sphere to form the pure purple opal.