Al2O3 changes from transparent to a range of intense colours depending on the chemical impurities present. In blue sapphire, Fe and Ti are incorporated; however, the chemical process that gives rise to the colour has long been debated. Atomistic modelling identifies charge transfer from Ti(III) to Fe(III) as being responsible for the characteristic blue appearance.
Bristow, J. K., Parker, S. C., A. Catlow, C. R., Woodley, S. M., & Walsh, A. (2013). Microscopic origin of the optical processes in blue sapphire. Chemical Communications, 49(46), 5259-5261. https://doi.org/10.1039/c3cc41506f