Obsidian

Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth.

It commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) causes a high viscosity, which, upon rapid cooling, results in a natural glass forming from the lava. The inhibition of atomic diffusion through this highly viscous lava explains the lack of crystal growth.

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