I knew pretty much immediately what it was, first I held it to a regular light then turned it out to see if it was just glow in the dark paint, it didn't glow, then I checked it with a UV flashlight in the van and it lit up brightly finally with the Geiger counter when I got home. They put radium paint on lots of things before they began to appreciate the dangers of it like clock faces and hands, dials and markers for aircraft. Radium paint is radioluminescent and glows all on it's own no need to shine a light on it to charge it up but over time the phosphors(the part that glows) wear out and take more energy than the radium can supply to glow, they show under UV though. I collect all things atomic