Obsidian is a type of igneous rock created when felsic magma cools and hardens before it can form a crystalline structure. It is characterized by its smooth texture and extremely sharp edges.
Obsidian is typically an extrusive type of volcanic glass, although it may form intrusively on the edges of dikes or sills. Its molecular structure allows it to be sharpened into extremely precise edges for use in modern surgical applications, and it was commonly used as a material for tools and weapons throughout history. Obsidian is chemically unstable and may crystallize unevenly over time, which may result in sporadic changes in color throughout the rock.