Professor Mendoza working a piece of obsidian. Flintknapping is usually done in a seated position with the heavy leather draped over a knee. It is helpful to have a thumb hole located near an edge to help grasp the obsidian. Obsidian flakes can be sharper than the sharpest steel razor. They can cut flesh like butter.
Obsidian can hold the sharpest edge known.
Obsidian is a volcanic glass. It is similar to man made glass, only sharper. It can hold an edge as thin as two microns, however, the obsidian edges wear quickly, and must be resharpened. Fortunately, obsidian is easily worked. It has a conchoidal fracture, that can produce large, usable flakes, or tiny chips for edgework.