With this method, values of molar absorptivity are not very accurate but provide a working assumption for our preliminary study. We recognize that further investigation is needed to fully understand the color in blue spinel. A Gaussian decomposition of spectra can improve the precision of these values. Nevertheless, it is apparent that in spinel, Co2+ is approximately 20 times more efficient at absorbing light, and thus creating color, than Fe2+ (consistent with D’Ippolito et al., 2015).