Glass is that mysterious translucent substance of what is essentially super-heated silica sand. Although details about the history of glass and glass making are still disputed, the earliest use of glass was undoubtedly that of the natural glass called obsidian. Obsidian is a natural byproduct of volcanic eruptions and it was prized by prehistoric societies the world over for its shiny black, orange, gray or green beauty, its sharp edges, and its workability.
Obsidian was used to make stone tools at least as early as the Middle Paleolithic, at sites such as Kaletepe Deresi 3 in Turkey near an obsidian outcrop, and the Upper Paleolithic Ortvale Klde site in Georgia, where researchers believe obsidian use helps underline a difference between Neanderthal and early modern human behaviors.
By the way, lightning strikes in sandy soils also create glass, called fulgurites, which occasionally turn up in archaeological sites.
Intentional glass making involves the superheating of crushed quartzite sand to produce a hot liquid, which is then allowed to cool to the clear, hard substance you recognize when you stare out the windows in your house or drink from a glass or place flowers in a vase, but that's the next step in the evolution of glass making.
A Bibliography of Glass Making has been assembled for this project.