Mohs Scale of Mineral Hardness

There are many systems used to measure hardness, which is defined several different ways. Gemstones and other minerals are ranked according to their Mohs hardness. Mohs hardness refers to a material's ability to resist abrasion or scratching. Note that a hard gem or mineral is not automatically tough or durable.

About the Mohs Scale of Mineral Hardness

The Moh's (Mohs) scale of hardness is the most common method used to rank gemstones and minerals according to hardness. Devised by German mineralogist Friedrich Moh in 1812, this scale grades minerals on a scale from 1 (very soft) to 10 (very hard). Because the Mohs scale is a relative scale, the difference between the hardness of a diamond and that of a ruby is much greater than the difference in hardness between calcite and gypsum. As an example, diamond (10) is about 4-5 times harder than corundum (9), which is about 2 times harder than topaz (8). Individual samples of a mineral may have slightly different Mohs ratings, but they will be near the same value. Half-numbers are used for in-between hardness ratings.

How to Use the Mohs Scale

A mineral with a given hardness rating will scratch other minerals of the same hardness and all samples with lower hardness ratings. As an example, if you can scratch a sample with a fingernail, you know its hardness is less than 2.5. If you can scratch a sample with a steel file, but not with a fingernail, you know its hardness is between 2.5 and 7.5. 

Gems are examples of minerals. Gold, silver, and platinum are all relatively soft, with Mohs ratings between 2.5-4. Since gems can scratch each other and their settings, each piece of gemstone jewelry should be wrapped separately in silk or paper. Also, be wary of commercial cleaners, as they may contain abrasives that could damage jewelry.

There are a few common household items on the basic Mohs scale to give you an idea of how hard gems and minerals really are and for use in testing hardness yourself.

Mohs Scale of Hardness

Mohs Scale History

While the modern Mohs scale was described by Friedrich Mohs, the scratch test has been in use for at least two thousand years. Aristotle's successor, Theophrastus, described the test around 300 BC in his treatise On Stones. Pliny the Elder outlined a similar test in Naturalis Historia, circa AD 77.

Other Hardness Scales

The Mohs scale is only one of a number of scales used to assess mineral hardness. Others include the Vickers scale, Brinell scale, Rockwell scale, Meyer hardness test, and Knoop hardness test. While the Mohs test gauges hardness based on a scratch test, the Brinell and Vickers scales are based on how easily a material can be dented. The Brinell and Vickers scales are especially useful when comparing the hardness values of metals and their alloys.

Sources

• Cordua, William S. (1990). "The Hardness of Minerals and Rocks". Lapidary Digest.
• Geels, Kay. "The True Microstructure of Materials". Materialographic Preparation from Sorby to the Present. Struers A/S. Copenhagen, Denmark.
• Mukherjee, Swapna (2012). Applied Mineralogy: Applications in Industry and Environment. Springer Science & Business Media. ISBN 978-94-007-1162-4.
• Samsonov, G.V., ed. (1968). "Mechanical Properties of the Elements". Handbook of the Physicochemical Properties of the Elements. New York: IFI-Plenum. doi:10.1007/978-1-4684-6066-7. ISBN 978-1-4684-6068-1.
• Smith, R.L.; Sandland, G.E. (1992). "An Accurate Method of Determining the Hardness of Metals, with Particular Reference to Those of a High Degree of Hardness". Proceedings of the Institution of Mechanical Engineers. Vol. I. pp. 623–641.