Obsidian is an extrusive igneous rock formed when a felsic lava erupts from a volcano and cools too quickly to allow crystals to form, resulting in a glass. Obsidian ranges in color from clear to green to most commonly black.
Obsidian is usually 70% or more SiO2 and similar in composition to granite or rhyolite. The mineral obsidian consists of relatively pure SiO2 (the same as quartz), but of course is a non-crystalline glass.
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