Or luminous gems: https://en.wikipedia.org/wiki/Luminous_gemstones#Mineralogical_luminosity https://en.wikipedia.org/wiki/Luminous_gemstones#Mineralogic... (I'

Or luminous gems: https://en.wikipedia.org/wiki/Luminous_gemstones#Mineralogical_luminosity https://en.wikipedia.org/wiki/Luminous_gemstones#Mineralogic... (I'm not remotely an expert, but it seems like at least a possibility for an explanation, and it's a interesting and seemingly little-known class of phenomena.)