Mohs Scale: Measuring the Hardness of Minerals

The Mohs Scale is used to determine the hardness of solids, especially minerals. It is named after the German mineralogist Friedrich Mohs. He was born in 1773, and lived until 1839. During the course of his career he was also a mine engineer as well as a professor of Mineralogy in Gratz. He wrote a "Treatise on Mineralogy", which has become a classic in the field.

In 1801 he got a job in Austria working for a wealthy banker identifying minerals in the banker's collection. In the course of this work he developed a scale to assign levels of hardness to various rocks and gems. He based his scale on the fact that some minerals could scratch others. If one mineral could scratch another and could not be scratched by it, he reasoned that it must be harder. He found that the softest mineral was talc and the one that scratched all of the others was the diamond. He gave his scale ten points, 1 being talc and 10 representing the hardness of diamonds. The other gemstones were assigned a number in between based on its relative hardness. However the scale does not reflect the fact that some minerals are many times harder. For example, Diamonds are four times harder than Corundum.

The scale he developed is still in general use today:

  1. Talc - easily scratched even by a fingernail
  2. Gypsum - barely scratched using a fingernail
  3. Calcite - will scratch a copper coin. It may also be scratched by the coin
  4. Fluorite - Can scratch a copper coin, but not scratched by a copper coin, also will not scratch glass
  5. Apatite - barely scratches glass, is also easily scratched by a knife
  6. Orthoclase - easily scratches glass and is barely scratched by a metal file
  7. Quartz - so hard that it is not scratched by a file
  8. Topaz - only corundum and diamonds can scratch it
  9. Corundum - second only to diamonds, it is scratched only by diamonds
  10. Diamond - scratched only by another diamond

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Interesting Fact:

The average fingernail has a hardness of about 2.5 on the Mohs scale.

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