Black light thundereggs!

The mineral in thundereggs that glows under black light is typically chalcedony, a type of microcrystalline quartz. When exposed to ultraviolet (UV) light, such as a black light, some chalcedony in thundereggs can fluoresce due to trace amounts of impurities like uranium or organic compounds. This fluorescence often appears as a greenish or yellowish glow, though the exact color can vary depending on the specific impurities present.
Thundereggs, which are volcanic rocks often filled with agate, chalcedony, or jasper, are prized by collectors for their hidden beauty, and this glowing property adds to their allure.