Mineralogy
"Hardness" is determined by comparison with other minerals. In the Mohs scale, a standard set of minerals are numbered in order of increasing hardness from 1 (talc) to 10 (diamond). A harder mineral will scratch a softer, so an unknown mineral can be placed in this scale by which minerals it scratches and which scratch it. A few minerals such as calcite and kyanite have a hardness that depends significantly on direction. Hardness can also be measured on an absolute scale using a sclerometer; compared to the absolute scale, the Mohs scale is nonlinear.
Hardness comparison
There are a large number of hardness testing methods available (e.g. Vickers, Brinell, Rockwell, Meyer and Leeb). Although it is impossible in many cases to give an exact conversion, it is possible to give an approximate material-specific comparison table e.g. for steels.