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.
https://github.com/davepinch/pinchy.cc/blob/master/content/topics/geology/rocks/obsidian/wikipedia/en.wikipedia.org.md