Obsidian
Obsidian is a natural volcanic glass found as a kind of igneous rock. It is produced when lava high in silica (SiO2) cools rapidly, and solidifies without 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]
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[change] Origin and properties
Obsidian is mineral-like, but not a true mineral because as a glass it is not crystalline. Obsidian is dark in color similar to 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.
Obsidian is metastable at the earth's surface: over time the glass becomes fine-grained crystals. So, no obsidian 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 it in 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 few samples are nearly colorless. In some stones, inclusions of small crystals produce a snowflake pattern (snowflake obsidian). It may contain patterns of gas which produce 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 hand axes, sharp blades or arrowheads. Like all glass and flint, obsidian breaks with a characteristic shell-shaped fracture.
Pre-Columbian Mesoamericans' used obsidian a lot. It was carved and worked for tools and decorative objects. It was also polished to create early mirrors. 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 edge.
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 (unworked lumps) 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 (high silicate) 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
- ↑ Obsidian is found in rhyolitic lava flows known as obsidian flows, where cooling of the lava is rapid.
- ↑ Primitive technology: a book of Earth skills David Wescott
- ↑ "Perlite - Mineral Deposit Profiles, B.C. Geological Survey". http://www.em.gov.bc.ca/mining/GeolSurv/MetallicMinerals/MineralDepositProfiles/profiles/r12.htm. Retrieved 2007-11-20.
- ↑ Obsidian was also used on Rapa Nui (Easter island) for edged tools such as Mataia and the pupils of the eyes of their Moai (statues).
- ↑ Buck B.A. 1982. Ancient technology in contemporary surgery. The Western Journal of Medicine, 136, 265-269
- ↑ "A comparison of obsidian and surgical steel scalpe...[Plast Reconstr Surg. 1993 - PubMed Result"]. http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=Retrieve&db=PubMed&list_uids=8415970&dopt=Abstract. Retrieved 2007-11-20.
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