Black Obsidian Mirror.
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.