Obsidian (pronounced ub-SIH-dee-in) is a natural glass that forms from volcanic activity. The gemstone is made of the same minerals as granite, but cooled so quickly that the minerals did not have time to crystallize. Obsidian is somewhat fragile, and is only slightly harder than window glass. It scratches easily and sharp blows can crack it. Do the lack of crystalline structure, it fractures in sharp curves, hence obsidian was used to make sharp tools as early as 21,000 B.C.
Clear obsidian contains very few opaque impurities or microscopic mineral crystals. Other colors generally result from tiny crystals of feldspar, or inclusions of iron oxide or a wide variety of other minerals that may be present in volcanic flows.
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