Dirt in certain places has the right mix of minerals to melt at much lower temperatures than sand alone, and would have made melted lumps under large campfires. The source material might have been natural, or the side-effect of some other industry, such as slag from smelting.

You can imagine the sequence of steps: Kids playing with sticks pulling out the red-hot gooey mess. Trying to make the gooey things solidify into interesting shapes. Realising the results are durable and shiny, if a bit ugly in colour. Then experimenting with mixing different kinds of dirt and clay to change the colours. Eventually discovering that translucent is possible, then transparent, etc...

https://en.wikipedia.org/wiki/History_of_glass