The Rockwell scale is a hardness scale used to measure the hardness of metals. It was developed by Stanley P. Rockwell in the 1920s and is used to measure the hardness of metals, plastics, and other materials. The Rockwell scale is based on a system of indentation tests that measure the depth of an indentation made by a diamond cone or ball indenter pressed into the material being tested. The resulting number indicates how hard the material is, with higher numbers indicating harder materials.