1. a hard, dark, glasslike volcanic rock formed by the rapid solidification of lava without crystallization.
Word Originearly 17th century (as an adjective): from Latin obsidianus, variant of obsianus, from Obsius, the name (in Pliny) of the discoverer of a similar stone.
Obsidian is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock. Wikipedia