Ah, found it, a relatively long read but it gives the complete story from beginning to end !! Leftover drain field media (clay balls) painted etc... & made to look like marbles, they're are still a lot of them around.
LINK : https://www.allaboutmarbles.com/viewtopic.php?f=7&t=51109&p=368324&hilit=clay+star+marbles#p368324