Obsidian
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Obsidian is a black, glass-like rock that forms when lava cools quickly. Broken rocks have sharp edges. Ancients used it for blades in knives and tools.
Kavanaugh, Pocket Naturalist
Sources
- Kavanaugh, James. Pocket Naturalist - Geology - An Introduction to familiar Rocks, Minerals, Gemstones & Fossils, Waterford Press: Arizona (2000)