Just been reading through the posts on the marble side of glass and yes, they are collectable, or at least here they are as I've mentioned before, the Teign Valley Glass works, museum, House Of Marbles is just down the road and they do have millions of them in varying forms and sizes. Haven't seen them being made as yet but the larger ones, 25mm and 50mm ones are done on a rolling machine and come off with striation lines on them which to me makes them look cheap but if you pick them over you can find very smooth ones. Old marbles around here are plentiful and fairly cheap, I bought 215 in a cardboard box at a boot sale this morning for £6 and these were the old traditional ones in sizes and varying colour inclusions from 15mm to 30mm and all between. I will certainly be looking for more and from time to time I still keep turning up the odd marble when working in my garden.