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[[Image:Obsidian.jpg|thumb|300px]]

[[Image:Obsidian.jpg|thumb|300px]]

'''Obsidian''' is dense [[volcano|volcanic]] [[glass]], usually [[rhyolite]] in composition and typically [[black]] in color. Compared with window glass, obsidian is rich in [[iron]] and [[magnesium]]; tiny (<.005 mm) crystals of iron oxide within the glass cause its dark color.  

'''Obsidian''' is dense [[volcano|volcanic]] [[glass]], usually [[rhyolite]] in composition and typically [[black]] in color. Compared with window glass, obsidian is rich in [[iron]] and [[magnesium]]; tiny (<.005&nbsp;mm) crystals of iron oxide within the glass cause its dark color.  

Obsidian is often formed in [[rhyolite]] [[lava]] flows where the lava cools so fast that [[crystal]]s do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth [[conchoidal]] (curved) shapes. The intersections of these fractures can form edges sharper than the finest [[steel]] blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades.<ref>http://volcanoes.usgs.gov/Products/Pglossary/obsidian.html</ref>

Obsidian is often formed in [[rhyolite]] [[lava]] flows where the lava cools so fast that [[crystal]]s do not have time to grow. Glass, unlike crystals, has no regular structure and therefore fractures in smooth [[conchoidal]] (curved) shapes. The intersections of these fractures can form edges sharper than the finest [[steel]] blades. For this reason, obsidian was used by many native cultures to make arrowheads and blades.<ref>http://volcanoes.usgs.gov/Products/Pglossary/obsidian.html</ref>