Hardness of materials has probably long been assessed by resistance to scratching or cutting. Relative hardness of minerals can be assessed by reference to the Mohs scale that ranks the ability of materials to resist scratching by another material.
The different methods and differently shaped indenters used by, for example, Brinell and Rockwell produce dissimilar responses of the material under test. To compare the hardness of two different samples, both must be measured using the same hardness scale, or a scale must be developed to convert from one measurement to the other.
The relationship of load to the area or depth of indentation is a measure of hardness, such we could found in Brinell, Rockwell, Vickers hardness testing. For practical and calibration reasons, each of these methods is divided into a range of scales, defined by a combination of applied load and indenter geometry.
originally found here:- http://www.ndtindia.org/portable-hardness.html