Obsidian is produced when felsic lava extruded from a volcano cools rapidly with minimal crystal growth.
Obsidian is hard and brittle; it therefore fractures with very sharp edges, which were used in the past in cutting and piercing tools, and it has been used experimentally as surgical scalpel blades.
Obsidian is a very protective stone, and is excellent for removing negativity. It is also excellent protection against psychic attacks. In particular obsidian protects the gentle from abuse, and is used to cut attachment cords to release one.