The Raman spectra were typical of spinel, with weak peaks at 405, 665, and 766 cm–1 (figure 10a; Fraas et al., 1973). The 405 cm–1 peak was 9 cm–1 wide, evidence that the analyzed spinels were natural and unheated (Krzemnicki, 2008; Saeseaw et al., 2009). However, this Raman signal of spinel was overwhelmed by luminescence with the two available excitation wavelengths (514 or 647 nm).