Obsidian

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This article is about volcanic glass. For other uses see obsidian (disambiguation).

Image:ObsidianOregon.jpg Image:Different rocks at Panum Crater-750px.jpg Obsidian is a type of naturally occurring glass, produced from volcanoes when a fluid felsic lava cools rapidly and freezes without sufficient time for crystal growth, for example by flowing into a body of water. It consists mainly of SiO₂ (silicon dioxide), 70% or more. Obsidian is mineral-like, but not a true mineral because it is not crystalline. Its composition is very similar to that of granite and rhyolite. It is sometimes classified as a mineraloid.

The color of obsidian varies depending on the presence of impurities. Iron and magnesium typically give the obsidian a dark green to brown to black color. The inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake 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. These bubbles can produce interesting effects such as a golden (sheen obsidian) or rainbow sheen (rainbow obsidian). Small nuggets of obsidian that have been naturally rounded and smoothed by wind and water are called "Apache tears." Obsidian is relatively soft with a typical hardness of 5 to 5.5. Its specific gravity is approximately 2.6.

Obsidian is commonly used for ornamental purposes and as a gemstone, for it possesses the peculiar property of presenting a different appearance according to the manner in which it is cut. When cut in one direction it is a beautiful jet black; when cut across another direction it is glistening gray.

Obsidian was highly valued in certain Stone Age cultures because, like flint, it can be fractured to produce sharp blades or arrowheads. Like all glass and some other types of naturally occurring rocks, obsidian breaks with a characteristic conchoidal fracture. It may also have been polished to create early mirrors. Image:Pig.snowobsidian.750pix.jpg

Image:Arrowhead.jpg Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated, including carved and worked obsidian for tools and decorative objects (see Obsidian use in Mesoamerica). The ancient Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a Maquahuitl, the weapon was capable of inflicting terrible injuries, combining the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon. Several eyewitness reports claim to witness it cleaving off opponents' limbs, heads, and even the head of an unlucky cavalry horse.

Obsidian is currently used in cardiac surgery, as well-crafted obsidian blades have a cutting edge much sharper than high-quality steel surgical scalpels, and may even be up to five times sharper, the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and less tissue trauma, which translates to faster healing and less scar tissue.

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