They basically put the American marble companies out of business. Modern marbles are not as collectible. Some were made with uranium in them and glow in the dark. Some antique marbles from China or Japan are made of porcelain. But then, anything round can be used as a marble, and generally has.
Some kids use the plastic balls from roll-on deodorant.
The thing is is, it's become harder to collect bottles because the good dump sites are pretty much sifted through already. Obviously, the last two are personal preferences. Like any other collectible, an item is worth only what you're willing to pay for it.