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.
Harshness
Classical timbre’ NLDs are roughness, spectral centroid, and spectral flux. While harmonicity and inharmonicity can also be considered NLDs, harshness differs from them, as well as from roughness, once it reckons for a distinguished perceptual audio feature expressed by the summary spectral periodicity. This feature is especially clear in single-pitch, single-note, musical audio, where the timbre of two different musical instruments can greatly differ in levels of harshness (e.g., the difference in harshness between a flute and a saxophone is evident). As it is supposed to be, harshness is independent of all others NLDs.