Microstructure and Play-of-Colour of Ethiopian Opal

The play-of-colour of Ethiopian opal is very different from the opal from the other localities.In this paper, we compared the microstructure differences of Ethiopian opal and analyzed the cause of play-of-colour. The results showed that Ethiopian opal has characteristic digit pattern and its porosity is relatively high. The Ethiopian opal is crystallinity opal in terms of phase composition.XRD, FTIR and Raman spectroscopy tests showed that Ethiopian opal is Opal-CT type opal with low temperature squamous quartz and low temperature square quartz (α-cristobalite). Scanning electron microscopy result revealed that Ethiopian opal is layered structure, with consisting of silica sphere (10-30 nm in diameter) arranged closely and irregular.Opal with blue and purple colour spot has sphere 80-100 nm in diameter.It concluded that the origin of the iridescence of Ethiopian opal is mainly caused by the interference of light from the internal layers, accompanied by a certain degree of Rayleigh scattering.