Mohs Hardness Scale

How Hard Is That Gemstone?

When mineralogists talk of hardness, they usually mean scratch-hardness--the mineral's resistance to being scratched.

The simplest method to measure the scratch-resistance of a mineral is using the Mohs Scale, devised by German mineralogist Friedrich Mohs in 1812.

Mohs selected ten standard minerals and arranged them on a scale of 1 to 10, with each one slightly harder than the preceding one. The softest mineral on the scale (number 1) is talc; the hardest mineral on the scale (number 10) is diamond, the world's hardest mineral.

All other minerals can be rated on this scale be determining which other minerals they will scratch and which they will not. Each mineral will scratch a softer mineral but be scratched by a harder one. Minerals of the same hardness will scratch each other..

Mohs scale is a relative scale, not an absolute scale. It only shows which gemstone is harder than another one--a relative comparison. The difference in hardness between any two points on the scale is different from the difference in hardness between any other two points. For example, a mineral with a hardness of four is NOT twice as hard as a mineral with a hardness of two nor half as hard as a mineral with a hardness of eight.

Before we had the scientific equipment available to examine a mineral optically, mineralogists relied much more heavily on the Mohs Scale scratch test than they do today. Now, with the more advanced testing equipment, the scratch-test for hardness is rarely used because of its imprecise testing of hardness and the potential for damaging the specimen being tested.

For rockhounds and for lapidarists, the Mohs Scale offers a general idea of rock and mineral hardness comparisons. Rockhounds may employ the test in the field while hunting for specimens. Lapidarists may find the test useful when determining how to cut, sand, and polish a particular rock/mineral specimen.