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.

People who prepare samples for electron microscopes will use glass "knives" to cut ultra-thin tissue samples. The clean cut edge of broken glass lets them make a very thin, smooth slice of their sample.

The character Raven in Neal Stephenson's novel Snow Crash uses a glass knife for this very same reason. Not the faster healing time, of course, but because it can cut through "cut proof" material that stops metal blades. Additionally, it can't be sensed by metal detectors. Good stuff. Fascinating.