Flame fusion synthetic spinel
If well ordered spinel crystal lattice receives heat above 800°C it starts to disorder. Disordering shows up in the PL spectrum so that emission peaks start to broaden, merge and shift. In other words, the spectra in Fig1. proves both the natural origin and the fact the stone has not received high temperature heat treatment for improving it's color or clarity.
Synthetic spinel manufactured by Verneuil method has been "heavily heat treated" during the flame fusion process, so the crystal lattice is never in very good order. While color variations and chromophores used in manufacturing of these synthetics are endless, they almost always have high enough chromium content for triggering the PL reaction. This fact applies even for colorless variation. Disorder of the crystal lattice broadens the main peak (the so called zero phonon line) significantly and also shifts it's position about 4-5 nm towards IR. Minor peaks have been broadened and merged. Please note this kind of spectrum is NOT indicative for Verneuil method as it is possible to obtain similar results from natural heat treated spinel as well. It is mandatory to continue tests with refractometer and microscope for making the final decision. It is also worth noting that chrome bearing tourmaline has a very similar PL spectrum. It is a good idea to perform Raman fingerprint analysis to make sure one is looking at spinel instead of tourmaline.
Figure 2: PL (GemmoRaman-532nm) spectra of synthetic spinel, flame fusion method (#142, #127 and #129 are shifted vertically for clarity)