Obsidian was highly valued in certain Stone Age cultures because, like flint, it could be fractured to produce sharp blades or arrowheads. Obsidian can also be found in the following countries: Armenia, Turkey, Italy, Mexico, Greece and Scotland.
US states including Arizona, Colorado, Texas, Utah, and Idaho. Yellowstone National Park has a mountainside containing much obsidian located between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western Among other places, large obsidan flows are found within the calderas of Newberry Volcano and Medicine Lake Volcano in the Cascade Range of western North America, and at Inyo Craters east of the Sierra Nevada in California. Obsidian can be found in many locations around the world which have experienced rhyolitic eruptions. These bubbles can produce interesting effects such as a golden sheen ( sheen obsidian) or a rainbow sheen ( rainbow obsidian). It may contain patterns of gas bubbles remaining from the lava flow, aligned along layers created as the molten rock was flowing before being cooled. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern ( snowflake obsidian). Iron and magnesium typically give the obsidian a dark green to brown to black color.
Pure obsidian is usually dark in appearance, though the color varies depending on the presence of impurities. Tektites were once thought by many to be obsidian produced by lunar volcanic eruptions, though few scientists now adhere to this hypothesis. Obsidian has a low water content when fresh, typically less than 1% water by weight, but becomes progressively hydrated when exposed to groundwater, forming perlite. This breakdown of obsidian is accelerated by the presence of water. Because obsidian is metastable at the earth's surface (over time the glass becomes fine-grained mineral crystals), no obsidian has been found that is older than Cretaceous age. Crystalline rocks with obsidian's composition include granite and rhyolite. Obsidian consists mainly of SiO 2 ( silicon dioxide), usually 70% or more. Though obsidian is dark in color similar to mafic rocks such as basalt, obsidian's composition is extremely felsic. It is sometimes classified as a mineraloid. Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline in addition, its composition is too complex to comprise a single mineral.
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