Before we learned the secret of glassmaking, nature was the only maker of glass.
Nature makes glass in two different ways.
One way is when lightning strikes sand, the heat fuses the sand into long, slender glass tubes called fulgurites. The other way is from the terrific heat of a volcanic eruption. Sometimes the eruption fuses rocks and sand into a glass called obsidian. We don't know exactly how man discovered how to make glass, but we do know that it was originally in the form of a glaze. Heat was used but not until much trial and error.