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Obsidian looks like a rock but it is actually a dense volcanic glass. Most obsidian rock is black, although on occasion it can be other colors. Obsidian forms in lava flows where the lava cools so fast that crystals do not have time to grow.

Find out more from the

And if you’re in the US, you can go visit an Obsidian Dome at Mammoth Mountain in California:
USGS Volcanoes

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