a large specimen of Snowflake obsidian
|Composition||70–75% SiO2, plus MgO, Fe3O4|
Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.It is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. Commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerisation degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerised lava explains the lack of crystal growth. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.