Once upon a time, I took all my marbles into a closet, and I started pulling them out according to how much they glowed under the blacklight.
I'd take out all the brightest ones and put them in one box.
Then after my eyes had a chance to adjust and those other bright ones were gone, some of the remaining ones started to look bright. So I would pull those out.
And then repeat the process.
When I got down to about level 4, I started seeing some really neat colors in the glow. Like up to four different colors in one marble.
Blue glow can come from calcium.
I ended up with a tan on my wrist from how much I used the blacklight. Don't use it as much as I did! hehe