obsidian

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 ]

☞ 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 ]