obsidian Meaning, Definition & Usage

  1. noun acid or granitic glass formed by the rapid cooling of lava without crystallization; usually dark, but transparent in thin pieces

WordNet

Ob*sid"i*an noun
Etymology
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.
Definitions
  1. (Min.) A kind of glass produced by volcanoes. It is usually of a black color, and opaque, except in thin splinters. ✍ 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.

Webster 1913