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The most common standard for rating gemstone hardness. Developed by Friedrich Mohs, a German minerologist, as an assessment of "scratch hardness" by ranking 10 different stones from hardest to softest. His list includes the following:
10. Diamond (hardest)
9. Sapphire
8. Topaz
7. Quartz
6. Orthoclase Feldspar
5. Apatite
4. Fluorspar
3. Calcite
2. Gypsum
1. Talc (softest)
Because the numbers are based on properties of actual stones, they are not equally distributed. So the difference between 10 (diamond) and 9 (sapphire) is not equal to the difference between any other two numbers. Very soft stones scratch more easily and some stones are too soft for jewelry.
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