The samples in this study displayed the typical Raman signal for spinel (top), with a strong continuum due to cobalt luminescence. But when the concentration in cobalt was too high, luminescence overwhelmed the signal for spinel. In some samples, spectra acquired using a 647 nm excitation showed additional luminescence peaks of chromium (the so-called organ pipe spectrum, bottom). Under green laser excitation (514 nm), the sample reacted with a red luminescence (bottom photo).