Mohs Scale of Hardness

Identify Rocks & Minerals Using Hardness

Scientists use the Mohs scale to gauge the hardness of minerals to help identify them.
Scientists use the Mohs scale to gauge the hardness of minerals to help identify them. Gary Ombler, Getty Images

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

Hardness Example
10 diamond
9 corundum (ruby, sapphire)
8 beryl (emerald, aquamarine)
7.5 garnet
6.5-7.5 steel file
7.0 quartz (amethyst, citrine, agate)
6 feldspar (spectrolite)
5.5-6.5 most glass
5 apatite
4 fluorite
3 calcite, a penny
2.5 fingernail
2 gypsum
1 talc