The first knives used were created by simply fracturing rock- a new fracture being sharper then the original weather worn edge. Any broken rock can be used as a knife but some rocks were favored over others. Flint, chert, greenstone, and jasper were valued as materials that were extremely strong and held an edge for a relatively long time. Obsidian was valued for its keen glass-like edge, but less useful for some things because it is so brittle. other stones with more of a sandy quality were used like rasps rather then knives but were still important tools.
Although a basic flake of rock can be used for a variety of purposes, as time wore on people began shaping rocks for specific uses- arrowheads, axes, matates, and knife and spear points. Rocks were shaped by a process called knapping- breaking small pieces of stone away from the core with a hammerstone. The flakes could then be pressure flaked with a small harder stone or antler into balanced knives, arrowheads or spears.
Knapping takes a long time to learn to do well- The skill is kept alive today by a few hands on anthropologists and archeologists, survivalists, and hobbyists, but was used commercially up until 1947 to make strikers for flintlock firearms.