definition of obsidian
A hard, dark, glass-like volcanic rock formed by the rapid solidification of lava without crystallization. Example sentences. 1  Hard stones such as obsidian and rock crystal were added to the repertoire of the stone-vase maker. The earliest mirrors were of obsidian, a black volcanic rock that could be carved and polished. 2  The presence of basalt and obsidian is a sign of volcanic activity and the instability of the earth's crust in this area.