Obsidian is a a natural glass. During volcanic eruptions the way in which the magma cools has an effect on the texture of igneous rocks. Three factors that affect the crystal size include: (1) the rate at which the magma cools; (2) the amount of silica present; and (3) the amount of dissolved gases in the magma.
The glassy texture seen in obsidian is caused by rapid cooling. The unordered ions are frozen before they could unite into an orderly crystalline structure. Obsidian was used by the native Americans to create cutting tools and arrowheads.
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