Frederick Mohs was a German mineralogist who devised a scale for determining the hardness of an unknown rock or mineral, which is often very useful in the identification process. This scale is a measure of a mineral's resistance to abrasion and is now known as the Mohs Scale of Hardness. The scale is defined by ten fairly common minerals (except for the diamond) of known hardness that are numerically ordered from the softest (1) to the hardest (10). Talc has a value of H=1, while the diamond has a value of H=10. The Mohs Scale of Hardness is based on the simple and observable fact that a harder material will scratch a softer one.

In 1802, Mohs moved to Austria, where he was employed in trying to identify the minerals in a private collection of a Banker. As part of this task, he started classifying minerals by their physical characteristics, in spite of their chemical composition, as had been done traditionally. This emphasis on physical characteristics was at odds with the prevailing chemical systematics. However, both Theophrastus and Pliny the Elder had compared the relative hardness of minerals known to them in the ancient world, including diamond and quartz. They knew that diamond could scratch quartz, so showing it to be harder. This became the basis of the famous scale hardness scale developed by Mohs. Minerals are now classified by chemical characteristics, but the physical properties are still useful in field examination.

Sources

• Cleveland, Cutler (Lead Author); Peter Saundry (Topic Editor). 2008. "Mohs, Frederick." In: Encyclopedia of Earth. Eds. Cutler J. Cleveland (Washington, D.C.: Environmental Information Coalition, National Council for Science and the Environment). [First published in the Encyclopedia of Earth September 15, 2006; Last revised December 1, 2008; Retrieved July 26, 2009]. 
• Wikipedia Contributors, Friedrich Mohs, Wikipedia The Free Encyclopedia, Accessed 26 July 2009.