Mohs hardness test is a qualitative ordinal scale used to determine the scratch resistance of minerals and other materials. The Mohs hardness scale is not defined by a mathematical formula but is a qualitative scale used to measure the scratch resistance of minerals based on a comparison with ten standard minerals. It ranks materials from 1 (softest) to 10 (hardest) based on their ability to visibly scratch one another. Each mineral on the scale can scratch those with a lower number and be scratched by those with a higher number. The test operates on the principle that a harder material will scratch a softer one, but not vice versa.
The scale comprises ten reference minerals: talc (1), gypsum (2), calcite (3), fluorite (4), apatite (5), orthoclase (6), quartz (7), topaz (8), corundum (9), and diamond (10). To perform the test, an unknown specimen is attempted to be scratched by materials of known hardness from the Mohs scale (or common objects like a fingernail at 2.5 or a copper penny at 3.0). If a material is scratched by one substance but not another, its hardness lies between those two points on the scale.
While invaluable for field geology and general mineral identification due to its simplicity, the Mohs scale is important to understand in context. It measures only scratch resistance and not other aspects of durability like toughness (resistance to breaking or chipping). Additionally, the scale is not linear; the difference in absolute hardness between consecutive numbers can vary significantly, with the jump from corundum to diamond representing a much greater increase in hardness than, for example, from talc to gypsum. Despite these limitations, its practical utility has ensured its continued use in mineralogy, gemology, and materials science.
