Micro-Raman spectroscopy was performed in the Department of Physics at the University of York using a Horiba Xplora System with a 532-nm semiconductor diode excitation laser providing a lateral spot extension of approximately 1 μm (data file S3). A 2400-T grating was used to obtain the highest spectral resolution of 1 cm−1 with an acquisition time of typically 1 s and 10 accumulations to minimize potential laser-induced damage to the sample and to reduce any interaction with the calculus matrix. After initial tests, it was decided that the relevant spectral range comprising the vibrational modes of interest was between 100 and 1600 cm−1. The characteristic blue color of the lazurite mineral particles in the context of otherwise translucent and colorless mineral particles enabled a rapid identification of their position at low magnification (10× objective lens) using transmission illumination with subsequent high-magnification imaging and spectral analysis (50× large working distance objective lens). The spectra were taken as single points on the largest particles that provided the best surface for the laser beam and that exhibited the largest distance from the calculus matrix. Micro-Raman spectroscopy was performed on the same slides of mounted archaeological blue particles as used for optical and SEM-EDS analysis.

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