Hardness is the result of the greater or lesser cohesion of minerals, or the strength of their chemical bonds. It is definable in terms of resistance to external stresses in one direction (scratching), in two (abrasion) or in three (penetration). The gem trade is mainly concerned with the first of these three. Given the difficulty of measuring hardness precisely, it is expressed in terms of an empirical scale consisting of ten sample minerals of increasing hardness, each one of which is capable of scratching the the preceding mineral, and being scratched in turn by the subsequent one. Minerals of the same hardness will not scratch each other. This is known as the Mohs’ scale. Frederick Mohs was a Viennese mineralogist (1773-1839). He was the first to set up a comparison scale using ten minerals of of different degrees of hardness, which is still widely in use. All minerals and gemstones known to us today are allocated on Mohs’ hardness scale.
5…Apatite (turquoise, opal)
6…Orthoclase (lapis lazuli, moonstone, opal)
7…Quartz (amethyst, citrine, garnet, peridot)
8…Topaz (emerald, aquamarine)
9…Corundum (ruby, sapphire)
The gemstones of hardness 1 and 2 are considered to be soft. Those of the degrees 3 to 5 medium hard, and those over hardness 6, hard. The scale is a relative hardness scale. It only shows which gemstone is harder than another one. Nothing is said about the increase of hardness within the scale. For example, while calcite may be three times harder than gypsum, and orthoclase is 6 times harder than apatite, corundum may be 7 times harder than quartz, but diamond is many thousand times harder than corundum!