Actually not as plain or sheet glass, Nature gifted us this gem in bubbly foamy solid state! In the high heat period of earth formation, melted rocks that contain highly silicoius matter were the main material of then-glass. They get exposed to surface sometimes and water content get evaporated resulting in sudden cooling of the melt. But due to the high viscosity, bubbles were trapped inside the cold melt, forming natural solid BUT foamy glass.
Glass was invented by Man in a different (Interesting!) way. Have you heard of Phoenicia? Founded in early 2500 BC, this Lebanon-based civilization (pre-Arab Caucasian by race) is famous for their commercial skill, economy depending on sea & invention of unique alphabet which is the basis of alphabet we are using today. Another thing Phoenicians had invented (or discovered?) — Glass!
Let me rephrase it from Pliny the Elder – some merchant Phoenicians who had a ship full of soda salt with them, landed on the bank of a river in Palestine. They were hungry & looking for something on which they will rest the cooking degs. Finding no other way, they took out few soda chunks from voyage cargo & placed under the pots. Then something MAGICAL happened – heat coming from fire melted soda, fused it with riverside sand & produced transparent Glass.
(1) Glass Chemistry by Werner Vogel
(2) Glass and Glass Manufacture by Percival Marson