Glass manufacturers used to add 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 under ultraviolet light (which might be hard to tell from these images).
Size is around ⅞ – 1” diameter. Yes they are slightly radioactive, but they're harmless.
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Uranium glass is glass which has had uranium, usually in oxide diuranate form, added to a glass mix before melting for coloration. The proportion usually varies from trace levels to about 2% uranium by weight, although some 20th-century pieces were made with up to 25% uranium.
The normal colour of uranium glass ranges from yellow to green depending on the oxidation state and concentration of the metal ions, although this may be altered by the addition of other elements as glass colorants. Uranium glass also fluoresces bright green under ultraviolet light and can register above background radiation on a sufficiently sensitive Geiger counter, although most pieces of uranium glass are considered to be harmless and only negligibly radioactive.
The most typical color of uranium glass is pale yellowish-green, which in the 1920s led to the nickname vaseline glass based on a perceived resemblance to the appearance of petroleum jelly as formulated and commercially sold at that time. Specialized collectors still define vaseline glass as transparent or semi-transparent uranium glass in this specific color.
Vaseline glass is now used as a synonym for any uranium glass, especially in the United States, but this usage is not universal. The term is sometimes carelessly applied to other types of glass based on certain aspects of their superficial appearance in normal light, regardless of actual uranium content which requires a blacklight test to verify the characteristic green fluorescence.
US production of uranium glasses ceased in the middle years of World War II because of the US government's confiscation of uranium supplies for the Manhattan Project from 1942 to 1958. After the restrictions in the United States were eased several firms resumed production of uranium glass, including Fenton, and Mosser; though uranium was still regulated as a strategic material. Following the Cold War, restrictions on uranium glass were completely lifted. During this time many older pieces entered the free market and new pieces continued to be produced in small quantities into the 2000's.