Obsidian

a volcanic rock which is formed by the rapid cooling of a molten rock with a composition of granite. It is generally glassy, black with a vitreous (glassy) lustre and in composition contains a lot of silica (SiO2).

Taken from Dictionary of Science

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Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools without crystal growth. Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition (high silica content) induces a high viscosity and polymerization degree of the lava. The inhibition of atomic diffusion through this highly viscous and polymerized lava explains the lack of crystal growth. Because of the lack of crystal structure, obsidian blade edges can reach almost molecular thinness, leading to its ancient use as projectile points, and its modern use as surgical scalpel blades.

Taken from Wikipedia