obsidian

/euhb sid"ee euhn/, n.
a volcanic glass similar in composition to granite, usually dark but transparent in thin pieces, and having a good conchoidal fracture.
[1350-1400; < L Obsidianus, printer's error for Obsianus pertaining to Obsius, the discoverer (according to Pliny) of a similar mineral in Ethiopia; r. ME obsianus < L; see -AN]

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Natural glass of volcanic origin that is formed by the rapid cooling of viscous lava.

It has a glassy lustre and is slightly harder than window glass. It is typically jet black, but the presence of hematite (iron oxide) produces red and brown varieties, and tiny gas bubbles may create a golden sheen. It is sometimes used as a semiprecious stone. Obsidian was used by American Indians and others for weapons, implements, tools, and ornaments, and by the ancient Aztecs and Greeks for mirrors.

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      natural glass of volcanic origin that is formed by the rapid cooling of viscous lava. Obsidian is extremely rich in silica (about 65 to 80 percent), is low in water, and has a chemical composition similar to rhyolite. Obsidian has a glassy lustre and is slightly harder than window glass. Though obsidian is typically jet-black in colour, the presence of hematite (iron oxide) produces red and brown varieties, and the inclusion of tiny gas bubbles may create a golden sheen. Other types with dark bands or mottling in gray, green, or yellow are also known.

      Obsidian generally contains less than 1 percent water by weight. Under high pressure at depth, rhyolitic lavas may contain up to 10 percent water, which helps to keep them fluid even at a low temperature. Eruption to the surface, where pressure is low, permits rapid escape of this volatile water and increases the viscosity of the melt. Increased viscosity impedes crystallization, and the lava solidifies as a glass.

      Different obsidians are composed of a variety of crystalline materials. Their abundant, closely spaced crystallites (microscopic embryonic crystal growths) are so numerous that the glass is opaque except on thin edges. Many samples of obsidian contain spherical clusters of radially arranged, needlelike crystals called spherulites. Microlites (tiny polarizing crystals) of feldspar and phenocrysts (large, well-formed crystals) of quartz may also be present.

      Most obsidian is associated with volcanic rocks and forms the upper portion of rhyolitic lava flows. It occurs less abundantly as thin edges of dikes and sills. The obsidians of Mount Hekla in Iceland, the Eolie Islands off the coast of Italy, and Obsidian Cliff in Yellowstone National Park, Wyoming, U.S., are all well-known occurrences.

      Obsidian was used by American Indians and many other primitive peoples for weapons, implements, tools, and ornaments and by the ancient Aztecs (Aztec) and Greeks for mirrors. Because of its conchoidal fracture (smooth curved surfaces and sharp edges), the sharpest stone artifacts were fashioned from obsidian; some of these, mostly arrowheads, have been dated by means of the hydration rinds that form on their exposed surfaces through time. Obsidian in attractive and variegated colours is sometimes used as a semiprecious stone.

      See also volcanic glass.

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Universalium. 2010.

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  • obsidian — OBSIDIÁN, obsidiane, s.n. Rocă vulcanică de culoare neagră sau brună cenuşie, cu înfăţişarea sticlei topite, care a fost folosită în epoca de piatră pentru confecţionarea armelor şi a uneltelor. [pr.: di an] – Din fr. obsidiane, obsidienne, lat.… …   Dicționar Român

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  • obsidian — ob*sid i*an, n. [L. Obsidianus lapis, so named, according to Pliny, after one Obsidius, who discovered it in Ethiopia: cf. F. obsidiane, obsidienne. The later editions of Pliny read Obsianus lapis, and Obsius, instead of Obsidianus lapis, and… …   The Collaborative International Dictionary of English

  • Obsidian — (Obsidianus lapis [weil Obsidius ihn zuerstin Äthiopien gefunden haben soll], auch Hahnstein, Glasachat, Glaslava, Glaszeolith), Mineral, erscheint amorphisch in derben, dichten Massen, in Kugeln, rundlichen Körnern u. Geröllen, porös, selten… …   Pierer's Universal-Lexikon

  • Obsidĭan — (Glaslava, Lavaglas), die wasserfreie oder nur bis 2 Proz. Wasser enthaltende glasartige Modifikation der Trachyte, meist schwarz (schwarze Glaslava) und grau, auch gelb, braun, rot, grün, selten blau, stark glasglänzend, durchsichtig bis… …   Meyers Großes Konversations-Lexikon

  • Obsidian — Obsidian, s. Gläser, natürliche …   Lexikon der gesamten Technik

  • Obsidian — Obsidiān, natürliches, vulkanisches Glas, durch rasche Abkühlung bes. kieselsäurereicher Schmelzmassen entstanden, samtschwarz oder braun, selten grau oder grün, von der chem. Zusammensetzung der Rhyolithe und Trachyte, bildet für sich Ströme,… …   Kleines Konversations-Lexikon

  • Obsidian — Obsidian, Pseudochrysolith, Fluolith, in Kugeln oder Körnern (Marekanit) vorkommendes glasartiges Mineral, von 2, 3 spec. Gewicht, von grauer, gelber, rother, brauner, meistens aber schwarzer Farbe, selten farblos u. beinahe wasserhell;… …   Herders Conversations-Lexikon

  • obsidian — (n.) dark, hard volcanic rock, 1650s, from L. obsidianus, misprint of Obsianus (lapis) (stone) of Obsius, name of a Roman alleged by Pliny to have found this rock in Ethiopia …   Etymology dictionary

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