Have only heard that manganese is the source for that orange glow . . . I never heard selenium connected with UV. Interesting. Must go light up my Cerises and other red slags tonight, since that's the red colorant in pretty much all machine-made marbles. If they glow orange I'll be shocked! Not like it's the first time I've received a Marble Shock, though.
I'll be interested to see if anyone knows where that light blue comes from. And occasionally I've seen lavender.
The last glass mentioned in that link article Steph posted, neodymium, is crazy. I have a "slag" made by Mike Edmonds out of neodymium glass, and I have amused my non-marble co-workers with it. I hand them this pretty, pale, icy blue slag (it's a little larger than 3/4) and ask them to keep holding it and walk outside with me. When we get outside, I tell them to look at the pretty blue marble again -- and the expression on their faces when they open their hands and find a lovely lavender marble is one of those "priceless" things. Pale blue in fluorescent light, lavender in incandescent and day light.