Mohs, Friedrich (1773-1839), was a German mineralogist who invented the Mohs scale for classifying the relative hardness of materials. The hardness of minerals may be tested by scratching one mineral with another. The harder mineral scratches the softer one. Mineralogists use a scale of hardness based on this principle.
The Mohs hardness scale lists 10 minerals from the softest to the hardest, numbered from 1 to 10. The hardness of other minerals is found by determining whether they scratch, or are scratched by, the minerals in the Mohs scale. For example, galena scratches gypsum (number 2), but is scratched by calcite (number 3). Therefore, galena’s hardness is 21/2—about halfway between that of gypsum and calcite.
The scale is as follows: talc 1; gypsum 2; calcite 3; fluorite 4; apatite 5; feldspar 6; quartz 7; topaz 8; corundum 9; diamond 10. A human fingernail measures 2+ on the scale; a copper coin about 3; and a pocketknife blade about 5+. As certain substances may only appear to be scratched when in contact with others, care must be taken to ensure that the apparent “scratch” is not just a mark which can be rubbed off.
Friedrich Mohs was born on Jan. 29, 1773, in Gernrode, near Quedlinburg, and was educated at the University of Halle. He was a professor at the universities of Graz and Vienna, both in Austria, and Freiburg, Germany. His written works include The Natural History System of Mineralogy (1821) and Treatise on Mineralogy (1825). Mohs died on Sept. 29, 1839, in Agordo, Italy, near Belluno.
