Obsidian is used in heart surgery , as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels , with the edge of the blade being only about 3 nm wide . Obsidian has been used for blades in surgery , as well-crafted obsidian blades have a cutting edge many times sharper than high-quality steel surgical scalpels , the cutting edge of the blade being only about 3 nanometers thick .