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From The Collaborative International Dictionary of English v.0.48:
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 Obsidius.] (Min.) A kind of glass produced by volcanoes. It is usually of a black color, and opaque, except in thin splinters. [1913 Webster] Note: In a thin section it often exhibits a fluidal structure, marked by the arrangement of microlites in the lines of the flow of the molten mass. [1913 Webster]