Obsidian

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Obsidian is an igneous rock that is very different than most other types. It is formed by the very rapid cooling of lava. When certain types of lava cool this fast, it makes volcanic glass, or obsidian. The rocks are very hard and shiny, and when they break, or fracture, they have very sharp edges. Since the Stone Age, humans have used obsidian to make cutting tools and tips for their arrows and spears.