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- Obsidian - When lava cools very quickly, it can harden into glassy, black obsidian. Native Americans made arrowheads out of this rock. It breaks easily into pieces with sharp edges
- Tuff - Tiny bits of lava and ash that get shot out of a volcano can become stuck together. When these particles cool and harden, they make tuff. Often other minerals fill in the holes in tuff. Tuff is pinkish tan in color.
- Peridotite - This dark green or black rock forms inside Earth's crust from magma. It contains a lot of iron. In South Africa, diamonds have been found in peridotite rocks.