If you're concerned that these Uranium Glass Marbles are simply standard marbles with phosphorescent material or somesuch, worry not! Amazon assures the consumer that they'll get a geiger counter going.
… A Ludlum Model 44-9 Geiger-Mueller probe connected to a Ludlum Model 3 when in direct contact will read several hundred cpm.
That said, a lot of the reviewers talk of using blacklight to see them, which does suggest an absolute minimum of radioactive material augmented by fluorescent paint. There's six in each pack of 5/8″ marbles.