Obsidian Rock Cluster

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock.

Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth. It is commonly found within the margins of rhyolitic lava flows known as obsidian flows. These flows have a high content of silica, which gives them a high viscosity. The high viscosity inhibits atomic diffusion through the lava, which inhibits nucleation of mineral crystals. Together with rapid cooling, this results in a natural glass forming from the lava.

Obsidian is hard, brittle, and amorphous; it therefore fractures with sharp edges. In the past, it was used to manufacture cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.