Obsidian is a naturally occurring volcanic glass formed as an extrusive igneous rock. It is produced when felsic lava extruded from a volcano cools rapidly with minimum crystal growth.
Obsidian is commonly found within the margins of rhyolitic lava flows known as obsidian flows, where the chemical composition induces a high viscosity and polymerization degree of the lava. Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and has been used experimentally as surgical scalpel blades.
The first archaeological evidence known of usage were made from within Kariandusi (Kenya) and other sites of the Acheulian age (beginning 1.5 million years previously) dated 700,000 BC, although the number of objects found at these sites were very low relative to the Neolithic.