Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. Obsidian is created when lava extruded from a volcano cools rapidly with minimal crystal growth. It is commonly found in obsidian flows. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Obsidian is hard and brittle and fractures with incredibly sharp edges. Although Obsidian is mineral-like, it is not a true mineral because as a glass it is not crystalline.