It's hard to believe but the sharpest knives that have ever been used in recent years were mounted with stone flakes made of obsidian
Obsidian is used by some surgeons for scalpel blades, 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.[34] Even the sharpest metal knife has a jagged, irregular blade when viewed under a strong enough microscope; when examined even under an electron microscope an obsidian blade is still smooth and even.
Good quality obsidian fractures down to single molecules which can produce a cutting edge 500 times sharper than the sharpest steel scalpel blade ("American Medical News", Nov. 2, 1984:21). On the cellular level an obsidian knife can cut between cells rather than tear the cells as a steel knife will do. A sharper cut will allow a wound to heal more rapidly with less scarring.
One study found that obsidian incisions produced narrower scars, fewer inflammatory cells, and less granulation tissue in a group of rats.
While they probably never practiced the art of surgery, our Paleolithic ancestors did use tools that have been extolled as more precise than the most modern metal scalpel.
Although very sharp the obsidian blade is very thin and cannot withstand lateral force on the blade. Surgeons must be very careful to cut only soft tissues with the obsidian scalpel. Attempting to cut or scrape bone could result in breakage, which could leave obsidian flakes inside the patient.