In the past, glass manufacturers added uranium to glass for color. Uranium has since become more strictly regulated, but the old uranium glass objects are still floating around. Because of the uranium content, these marbles glow bright green under ultraviolet light. * Size is around ⅞ – 1” diameter. * Some dents and manufacturing imperfections can be expected. * Slightly radioactive, but they're harmless. * Makes a great sample for uranium if you're collecting the elements This item pairs well with our ultraviolet LED flashlights.