This article is about a type of volcanic glass. For other uses see obsidian . Obsidian is a type of naturally-occurring glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly through the glass transition temperature and freezes without sufficient time for crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades. Origin and properties Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex to comprise a single mineral. It is sometimes classified as a mineraloid. Though obs

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