Obsidian, a natural volcanic glass, has been used for millennia as a favored tool material because of its capacity for unusually sharp edges and its ease of redress. Obsidian gains its sharp edges from the properties of the material itself. When struck a sharp blow, the force of the fracture travels through the material in a wave that cleaves off a portion of the glass. The resulting 120 degree curve forms a natural edge that is very similar to a hollow ground edge.
The result, if done properly, is a microscopic edge so thin and durable that it can cleanly slice the smallest and most delicate of materials, like cell walls. In comparison, Surgical steel polished to the same edge and thinness will collapse under the pressure applied during the cutting motion. In essence the inflexibility that makes obsidian so brittle is the property that makes it so much more desirable over steel at the microscopic level.