Obsidian (/əbˈsɪdi.ən, ɒb-/ əb-SID-ee-ən ob-) is a naturally occurring volcanic glass formed when lava extruded from a volcano cools rapidly with minimal crystal growth. It is an igneous rock.
Its beauty was wild and untrammeled and the undulating hills were covered with luxuriant grasses. Moscow, Idaho homesteader, 1880s