So, my source is a marble maker and says he never heard of Thorium being used but once read in an article on a company overseas that at been experimenting with it as a possible replacement for uranium. He expects it's possible for some of the glass by-product into a marble maker's hands.
Also, he noted that there might be some issues with the radioactivity of the Thorium ... so don't put it up to your eyes unless you're sure it's okay!
So, Richard, to add a little background and to put it together with what I learned just now: Marblemakers do a lot of glass recycling. They don't mix up all their glass from scratch with a formula anymore. The recycled glass from other sources could have included Thorium. My marble-making friend is not aware of it being done on a systematic basis.