Obsidian is a natural glass of volcanic origin formed at the time of solidification of the lava when cooled very quickly. It is usually black although it can have brown, green and reddish tones by the presence of magnetite or hematites (iron oxide). It may also contain spherical inclusions of other grayish minerals, a feature that gives it the name Obsidian Nevada. Gold or silver reflections are sometimes seen, the result of gas bubbles or pores that it may contain.
For their fracture and hardness properties (5-5.5 on the Mohs scale), ancient cultures used obsidian to craft tools, weapons, and ornamentation.
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