Obsidian

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

Obsidian is a glass, that can be found in nature as a sort of rock. It is produced when felsic lava comes out of a volcano, cools rapidly and freezes without sufficient time for crystal growth.^[1] Because there is no crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as arrowheads, and its modern use as surgical scalpel blades.^[2]

Contents 1 Origin and properties 2 Historical use 3 Occurrence 4 Current use 5 References 6 Other websites

[change] Origin and properties

Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline; in addition, its composition is too complex that it could be a single mineral. It is sometimes classified as a mineraloid. Obsidian is dark in color similar to mafic rocks such as basalt. It consists mainly of SiO₂ (silicon dioxide), usually 70% or more. Crystalline rocks with obsidian's composition include granite and rhyolite.

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. This breakdown of obsidian is accelerated by the presence of water. Obsidian has a low water content when fresh, typically less than 1% water by weight ^[3], but takes in water when exposed to groundwater.

Pure obsidian is usually dark in appearance, though the color varies depending on the presence of different materials. Iron and magnesium typically give the obsidian a dark green to brown to black color. A very few samples are nearly colorless. In some stones, the inclusion of small, white, radially clustered crystals of cristobalite in the black glass produce a 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 (sheen obsidian) or a rainbow sheen (rainbow obsidian).

[change] Historical use

Obsidian was valued in Stone Age cultures because, like flint, it could 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 was also polished to create early mirrors.

Pre-Columbian Mesoamericans' use of obsidian was extensive and sophisticated. It was carved and worked for tools and decorative objects. Mesoamericans also made a type of sword with obsidian blades mounted in a wooden body. Called a macuahuitl, the weapon could cause terrible injuries, because it combined the sharp cutting edge of an obsidian blade with the ragged cut of a serrated weapon.

Native American people traded obsidian throughout North America. Each volcano and in some cases each volcanic eruption produces a distinguishable type of obsidian. So archaeologists can trace the origins of a particular artifact. Similar tracing techniques have allowed obsidian to be identified in Greece also as coming from different islands in the Aegean Sea. Obsidian cores and blades were traded great distances inland from the coast.^[4]

Modern archaeologists have developed a dating system to calculate the age of Obsidian artifacts depending from the content of water in the object.

[change] Occurrence

Obsidian can be found in locations which have experienced rhyolitic eruptions. Obsidan flows which you can hike on 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. Yellowstone National Park has a mountainside containing obsidian between Mammoth Hot Springs and the Norris Geyser Basin, and deposits can be found in many other western US states including Arizona, Colorado, Texas, Utah, Oregon and Idaho. Obsidian can also be found in Armenia, Turkey, Italy, Mexico, Iceland, Greece and Scotland.

[change] Current use

Obsidian is used in heart surgery, as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels, with the edge of the blade being only about 3 nm wide ^[5]. Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope. When examined under an electron microscope an obsidian blade is still smooth and even. One study found that obsidian produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.^[6]

Obsidian is also used for ornamental purposes and as a gemstone. It possesses the 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.

[change] References

[change] Other websites

• USGS Obsidian photo glossary
• Jim Miller, Obsidian is Hot Stuff, Volcano World
• Obsidian: Mineral galleries
• Mindat with location data