What is the Mohs scale?

The Mohs scale is a standardized scale for evaluating the scratch resistance of a mineral. (Most gemstones are minerals.) It’s named after Viennese mineralogist Friedrich Mohs (1773-1839).
The Mohs scale is a relative scale—that is, it assigns a Mohs value based on a mineral’s relation to other minerals. A mineral with a higher designation will not scratch other minerals with the same rating, but will scratch those with a lower rating.
A diamond has a Mohs hardness of 10, the only gem to have this distinction. No other gemstone will scratch a diamond, but it can scratch any other gemstone.
The scratch test is an excellent field tool for geologists, as it doesn’t require carrying anything more than a small and inexpensive test kit. However, it’s seldom used by gemmologists. Mohs hardness alone is not always sufficient for identification, as several different gemstones have the same rating, and the test risks harm to the gem.
For example, both the labradorite in these Charleston earrings and the rainbow moonstone in our Crescent Moon earrings have the same rating of 6 to 6.5 on the Mohs scale.
Instead, gemmologists use optical examinations and other non-invasive tests to determine the identity of a gemstone.
The Mohs rating for a gemstone is good to know, but insufficient for a cutter, gem setter, or jeweller, who should also know about a gem’s brittleness and susceptibility to fracture or damage due to wear, among other things.
When identifying and grading a gemstone, a gemmologist will apply several non-invasive tests to evaluate both the identity and the quality of the gem. These tests must be done before a gemstone is set, as the gemstone cannot be properly evaluated once it’s in a setting.
“Gemstones of the World” by Walter Schumann, Sterling New York, Fifth Edition

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