Uranium glass, popular from the 1950s through the 1970s, gets its color from uranium dioxide mixed into the melt — usually 2 to 10 percent — and fluoresces bright green to orange under a blacklight, not just a dull glow-in-the-dark. Each vintage marble is unique, sold five to a velvet bag or fifteen to a display jar, and the seller states plainly that the trace radioactivity at that concentration is harmless.