It's the cooling of silica (really, any material) that makes it a glass, and even then, transparency in the visual wavelength is not automatically certain.
Maybe tektites? Natural glass formed when lightning meteorites strikes sand. I only remember the name because they share it with the jumpy spiders from Zelda
If you spent your days cooking with fire, and your nights watching it and warming yourself, you'd definitely start tossing anything you could find into it just to see what would happen. People did this every day and night for eons.
I think people just experimented a lot. Try enough random things, you're bound to come across cool chemistry every once in a while. If they figured out how to make really hot fire, that opens the path to "let's try making various things really hot to see what happens".
Of course, I know basically nothing about [pre]history or human development so I could be way off