Top: Uranium marbles under white light
Bottom Uranium marbles under an ultraviolet light (blacklight)
Image Credit: Mike Lucibella
Under an ultraviolet blacklight, these marbles give off a neon glow — a product of the small amounts of uranium found inside the spheres. In fact, these marbles contain about 2 to 3 percent uranium by weight.
The photons emitted from an ultraviolet light excite the electrons in uranium atoms. In turn, this change in energy level releases photons of a different energy that appear neon green or yellow.
So why aren't these marbles dangerous? It's all a matter of degree. Although the marbles emit beta particles and gamma rays (which are dangerous at high levels), they only give off a small number of these radioactive decay products. These marbles barely register on sensitive Geiger counters, and the radiation emitted barely creeps above the background radiation we experience everyday from natural sources.