What is the name of a natural volcanic glass, a rock used in the past to make it very sharp blades for spears and arrows

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Explanation:

Obsidian is a naturally occurring volcanic glassformed as an extrusive igneous rock. Obsidian is produced when felsic lavaextruded from a volcano cools rapidly with minimal crystal growth.

Obsidian

Obsidian was used to make knives, arrowheads, spearpoints, scrapers, and many other weapons and tools. Once these discoveries were made, obsidian quickly became the raw material of preference for producing almost any sharpobject.