Material Measures: The Mohs Scale of Mineral Hardness

An essential aspect of many material specification methods involves a determination of durability and resistance to abrasion. Developed over 200 years ago, the Mohs scale of mineral hardness offers a straightforward system for testing the toughness of materials—including minerals, rocks, and gemstones—that roughly measures of the resistance of a smooth surface to scratching or abrasion. Adapted from early methods used in antiquity, German Geologist Freidrich Mohs devised a qualitative scale for characterizing the scratch resistance of minerals, which has served as a valuable tool for testing material hardness since 1812. The information below outlines an overview of the Mohs scale of mineral hardness.

Scratch Resistance

The Mohs scale measures the hardness of a mineral by observing how its surface is scratched by another substance. In order to use the Mohs scale to measure material hardness, a sharp point of one material is placed on an unmarked surface of another material in an attempt to produce a scratch. “It compares the relative hardness of naturally occurring minerals,” says Kevin Matthews of HanStone Quartz. “In short—it is the ability of one mineral to scratch another. There are no measurements or tests that can be performed, other than scratching, to determine the Mohs value.” Thus, the ability of one material to scratch another determines its rating on the Mohs scale, and if two materials have an equivalent hardness they will be relatively ineffective at scratching one another.

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Comparative Scale

In order to assign a numerical value to the physical property of scratch resistance, materials are ranked along a scale of ten common minerals—including talc, gypsum, calcite, fluorite, apatite, orthoclase, quartz, topaz, corundum, and diamond. These minerals were selected based on availability and they have each already been assigned a hardness rating. “The scale is organized on a categorical listing from one to ten, with one being represented by talc, the softest mineral, and ten being diamond, which is the hardest mineral,” says Matthews.

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While other measures of material science offer more quantitative methods of comparing mineral hardness, the Mohs scale serves as a truly comparative assessment for juxtaposing the strengths of various minerals. “The fact that quartz ranks as a seven on the Mohs scale means that only materials that are equal or higher on the scale can scratch it,” says Matthews. “So, when we make the claim that our material is scratch resistant, we know that most metals all fall below five on the Mohs scale and will not scratch the quartz content of HanStone.” As is indicated by their ranking, the hardness of a material is measured against the scale by finding the hardest material that it can scratch, or the softest material that it can be scratched by. Thus, the scale comparatively conveys a material’s scratch resistance.

End Uses and Limitations

Having an understanding of a material’s hardness rating on the Mohs scale can help with deciding on its potential end uses, as it will aid in assessing the amount of wear and tear that can be sustained. “A product with a high Mohs rating generally relates to a product that can be used in a more high-traffic commercial environments,” says Nick Goodman of Atlas Concorde. “A product with a low Mohs hardness scale of a four or five would be suitable for lighter traffic residential uses but would not be considered an acceptable for use in a high traffic areas.”

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Moreover, the Mohs scale is purely ordinal and not linear, which means that the difference in hardness between each of the numbers on the scale is not the same. For example, though they respectively hold ratings of three and four on the Mohs scale, the minerals calcite and fluorite have a difference in hardness that measures approximately 25 percent, while diamond—with the highest rating of ten—is approximately 300-percent harder than corundum, which has a rating of nine. Additional limitations of the Mohs scale include irregularities in its procedure and its overall effectiveness for evaluating durability across applications.  “While Mohs can guide you to what can or cannot scratch the surface of a product, it doesn’t fully determine the overall durability and cannot be solely used to determine if it is applicable with a certain destination of use,” says Goodman. “However, it is still seen to be useful in the industry when utilizing in conjunction with other data about a particular product and its planned destination of use.”

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