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obsidian(ŏbsĭd`ēən), a volcanic glassglass,
hard substance, usually brittle and transparent, composed chiefly of silicates and an alkali fused at high temperature. Composition and Properties of Glass
..... Click the link for more information. , homogeneous in texture and having a low water content, with a vitreous luster and a conchoidal fracture. The color is commonly black, but may be some shade of red or brown, and cut sections sometimes appear to be green. Like other volcanic glasses, obsidian is a lavalava
, molten rock that erupts on the earth's surface, either on land or under the ocean, by a volcano or through a fissure. It solidifies into igneous rock that is also called lava.
..... Click the link for more information. that has cooled too quickly for the contained minerals to crystallize. In chemical composition it is rich in silica and similar to granite. It is favored by primitive peoples for knives, arrowheads, spearheads, and other weapons and tools.
(Latin Obsidianus lapis, “Obsidius’ stone”; according to Pliny the Elder, the rock was discovered in Ethiopia by a certain Obsidius, or more accurately, Obsius), a homogeneous vitreous volcanic rock, usually dark-colored (black, reddish black), with a marked conchoidal fracture. The hardness on Mohs’ scale is 5, and the density, is 2,500–2,600 kg/m3. The rock contains about 0.5 percent water.
Obsidian polishes well. It is formed upon hardening of the viscous varieties of rhyolite lava or rhyodacite lava. Certain varieties of obsidian are used as decorative stones. In the Stone Age and Bronze Age, obsidian was used to make arrowheads, spearheads, knives, and scrapers. At present it is used mainly as an expanding filler for lightweight concretes. Obsidian is found in areas of volcanic activity—for example, in Transcaucasia and on Kamchatka in the USSR, and also in Italy and Mexico.