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Obsidian is an igneous rock that is made from high silica magma, and cools so fast that it is unable to form crystals! Obsidian is often referred to as volcanic glass. -- Kate Borden

Igneous rocks are formed from the solidification of molten rock material. There are two basic types: 1) intrusive igneous rocks such as diorite, gabbro, granite and pegmatite that solidify below Earth's surface; and 2) extrusive igneous rocks such as andesite, basalt, obsidian, pumice, rhyolite and scoria that solidify on or above Earth's surface. Pictures and brief descriptions of some common igneous rock types are shown on this page. - Tucker Hale