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. 

Photos are representative of typical specimens. Some natural variation in size/shape should be expected.  

Details:
Approx. 7/8" x 1" Diameter
Specimen displayed in an acrylic case
Slightly radioactive, but they're harmless

By Stemcell Science Shop