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The Mohs Scale of Mineral Hardness

The Mohs scale is a qualitative tool used by geologists to determine the relative hardness of minerals. Developed by German geologist Friedrich Mohs in 1812, the scale ranks ten common minerals on a scale from 1 to 10. It is an ordinal scale, meaning it does not show the absolute or quantitative hardness of the minerals, but rather which mineral is capable of scratching another. The softest mineral on the scale, with a rank of 1, is talc. The hardest mineral, with a rank of 10, is diamond.

The method for using the Mohs scale is based on a simple scratch test. A mineral's hardness is determined by finding the hardest mineral it can scratch and the softest mineral that can scratch it. For instance, if an unknown mineral scratches fluorite (hardness 4) but is scratched by apatite (hardness 5), its hardness is between 4 and 5. Geologists often use common objects for a quick field test: a fingernail has a hardness of about 2.5, a copper penny is about 3.5, and a piece of steel is about 6.5. This makes the scale a practical and convenient tool for initial identification.

It is important to understand the limitations of the Mohs scale. The scale is not linear. For example, the difference in hardness between diamond (10) and corundum (9) is far greater than the difference between corundum and topaz (8). Diamond is many times harder than corundum. While the Mohs scale is excellent for quick field identification, more sophisticated instruments are required for measuring the absolute hardness of materials in industrial or scientific settings. Nevertheless, its simplicity has made it an enduring and widely used tool in geology for over two centuries.