A smooth, black, glass-like rock formed when lava cools very quickly.
From Latin "obsidianus (lapis)" meaning "(stone) of Obsius," possibly named after a Roman named Obsius who was said to have discovered a similar stone in Ethiopia. Over time the spelling settled into "obsidian."
Obsidian is basically natural volcanic glass, sharp enough that ancient people used it for blades and arrowheads. Modern surgeons sometimes use obsidian scalpels because they can be far sharper than steel.
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