Description

Obsidian is a type of naturally-occurring glass formed as an extrusive igneous rock.  It is produced when felsic lava erupted from a volcano, cools rapidly through the glass transition temperature and freezes without sufficient time for Crystal growth.  Because of the lack of Crystal structure, Obsidian blade edges can reach almost molecular thinness, leading to its ancient use of projectile points (arrow heads), and its modern use as surgical scalpel blades.
In some stones, the inclusion small, white, radially clustered crystals of cristobalite in the black glass produce a blotchy or snowflake pattern (snowflake obsidian).