obsidian rock
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A naturally occurring volcanic glass formed from cooling lava.
Obsidian is hard and brittle; therefore it fractures with very sharp edges, which had been used in the past for cutting and piercing tools, and are still used as surgical scalpel blades.
Obsidian is also used for ornamental purposes and as a gemstone. It appears differently according to the direction in which it is cut: when cut in one direction it is jet black; in another it is glistening gray.
