obsidian

noun

1. a volcanic glass similar in composition to granite, usually dark but transparent in thin pieces, and having a good conchoidal fracture.

noun

1. a dark volcanic glass formed by very rapid solidification of lavaAlso called: Iceland agate

n.“dark, hard volcanic rock,” 1650s, from Latin obsidianus, misprint of Obsianus (lapis) “(stone) of Obsius,” name of a Roman alleged by Pliny to have found this rock in Ethiopia.

1. A shiny, usually black, volcanic glass. Obsidian forms above ground from lava that is similar in composition to the magma from which granite forms underground, but cools so quickly that minerals do not have a chance to form within it.