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)
6. Orthoclase Feldspar
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.