Obsidian is volcanic glass formed when lava cools so quickly that crystals cannot grow.
It usually develops from silica-rich lava, especially rhyolitic lava, whose high viscosity helps prevent atoms from arranging into large mineral crystals. The resulting material has a glassy texture and commonly appears black, though red, brown, green, and banded varieties also occur.
Obsidian breaks with a characteristic conchoidal fracture, producing curved surfaces and extremely sharp edges. Indigenous peoples around the world shaped it into knives, arrowheads, scrapers, and other tools. Modern scientists also use obsidian hydration and chemical composition in archaeological studies.
Obsidian is a rock rather than a single mineral. It is also different from pumice: pumice is highly vesicular and full of gas cavities, while obsidian is comparatively dense and glassy.