24 Obsidian

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Obsidian is an <a href="http://geology.com/rocks/igneous-rocks.shtml">igneous rock</a> that forms when molten rock material cools so rapidly that atoms are unable to arrange themselves into a crystalline structure. The result is a volcanic glass with a smooth uniform texture that breaks with a conchoidal fracture (see photo on right).</p>

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<img src="/19ab56fb-9f61-51cf-b1d1-1e4520de1bec.jpg"/></p>