When is Moh's scale used?

Moh's scale - is used to measure the hardness of substances. The scale was introduced by the German mineralogist Friedrich Mohs in 1812. It is based on a classification of minerals. These are arranged in a table so that each of them is hard enough to scratch those below it, but not hard enough to scratch any of those above it. The scale in order of decreasing hardness is: 10, diamond; 9, carborundum and sapphire; 8, topaz; 7, quartz; 6, orthoclase; 5, apatite; 4, fluorite; 3, calcite; 2, gypsum; 1, talc. The divisions are, however, not equal. For instance, on an absolute scale, the difference in hardness between 10 (diamond) and 9 (carborundum) is four times greater than the difference between 9 (carborundum) and 1 (talc). The 10 minerals thus provide a table of reference, against which the comparative hardness of other minerals can be measured.