Answer: Uranium

Many vintage marbles have a curious property that delights kids and adults alike: if you shine a black light (or another source of intense ultraviolet light) on them, they fluoresce an absolutely brilliant green color.

Why do some vintage marbles shine brilliantly and some simply react to the presence of the black light like any other glass? The secret is the composition of the glass: many vintage marbles were manufactured with uranium infused glass. When illuminated by ultraviolet light, the glass shines an intense (and stereotypically “radioactive”) green.

But why would anyone add uranium, however small the amount, to glass marbles? It seems especially odd when you consider that back in the 1920s, nobody was walking around with a pocket black light to make their marbles glow—so the novel appearance of marbles in such a situation wouldn’t have even been a consideration.

The selling point wasn’t the glow (which is something enjoyed by modern collectors), but a much more subtle coloration. Uranium infused glass, commonly called “Vaseline glass” by collectors because of its slightly cloudy appearance, takes on a special appearance when exposed to the even smaller concentration of ultraviolet light rained down upon us by the sun. Uranium infused glass vases, bowls, prisms, and glass door panels took on a beautiful, almost opalescent emerald quality when sunlight hit them just right. Although the green tint was by far the most common, other variations include pale yellow and blue.

Uranium glass usage fell out of favor during the late 1930s and 1940s as the United States government snapped up all available uranium for nuclear research, development, and weapons construction purposes. Today, a small handful of marble companies still make small runs of marbles with uranium glass, and an even smaller number of glass artisans still craft vases, bowls, and decorative objects with the curious glass. If you’re worried about the safety of uranium glass trinkets, don’t be. While some rare pieces have higher than normal amounts of uranium content, the vast majority of uranium glass is considered negligibly radioactive and requires very sensitive equipment to even detect its radioactivity.