Now, I'm not sure about the realism of such a thing, since I haven't studied crystal lattices or anything, but...:
Name: Kessavych crystal
Discovered by an old Nepleslian kook famous for his artificial mineral gardens, the Kessavych crystal formation is similar in structure and, indeed, in concept to phosphorescence - the crystals absorb light gradually as they enter activated states. However, rather than releasing it slowly, as with conventional phosphorescence, the Kessavych crystals, upon reaching a certain point, release it all at once, delivering a blinding flash over a few seconds. As with any man with any sort of power or money on Nepleslia, Kessavych had connections to the Blacks; he regularly paid his protection money, and they kept his complex safe. It seems, now, that he might not have to pay it for a while...