Obsidian is classified as a volcanic glass, specifically an extrusive igneous rock. It forms when lava cools rapidly, resulting in a glassy texture due to the lack of time for crystal growth.
The rapid cooling occurs when lava comes into contact with water or air, causing it to solidify before forming crystals. This unique formation process gives obsidian its smooth, shiny appearance and sharp edges, making it useful for tools and ornaments throughout history.
Unlike many other igneous rocks that typically crystallize and display a granular texture, obsidian’s lack of crystalline structure is what sets it apart in the geological community. Its composition primarily includes silica, often with varying amounts of iron and magnesium, which can give it different colors and patterns.