Obsidian knife, probably used for ritual or ceremonial activities, found in Tarascan territory (Museo Michoacano, Morelia. Photo by Eduardo Williams)
[Image Credit: Foundation for the Advancement of Mesoamerican Studies]
Ancient Indian hunters may actually know something when they use obsidian (a naturally-occurring glass formed from volcano lava) as knives: a scientific study showed that obsidian blades are sharper than surgical scalpels:
Obsidian is used in cardiac surgery, as well-crafted obsidian blades have a cutting edge up to five times sharper than high-quality steel surgical scalpels, with the edge of the blade reaching veritable molecular thinness. It produces a cleaner cut and less tissue trauma, which translates to faster healing and less scar tissue.
Source: Obsidian [wiki] - Thanks eef!