New Insights on the Origin of the Blue Photoluminescence of Natural Opal Through Raman Spectroscopy

The blue luminescence commonly observed in natural opal (amorphous, hydrated silica) under 3.40 eV light remains poorly understood. Previous studies, lacking evidence of extrinsic activators, have attributed this emission to intrinsic defects. Raman spectroscopy has enabled the quantification of Qⁿ units a reliable indicator of silica polymerization degree. Thus, we investigated possible correlation between blue luminescence spectra and opal polymerization degree. We used Raman spectroscopy to measure the proportions of Qⁿ units on eight natural samples. We then decomposed the blue luminescence spectra of the same samples and observed 3 components at 2.49, 2.73, and 3.00 eV (respectively 498, 454, and 413 nm). The intensity of each contribution exhibits a linear relationship with the proportions of Q³, Q⁴, and Q² centers, respectively. As Q⁴ alone cannot induce luminescence, we propose that the 2.73 eV emission band is linked to an oxygen deficiency center.