Volcanic glass is a natural glass formed by the rapid cooling of molten lava, preventing the formation of crystals. Obsidian is a common example. Volcanic glass is significant in petrology and archaeology for understanding the cooling history of volcanic eruptions, the composition of magmas, and the use of volcanic glass in prehistoric tools.
Source Dingwell, D. B. (1996). “Volcanic Glasses.” Reviews in Mineralogy and Geochemistry, 32(1), 69-106.