If you've ever heard a geologist or collector refer to a minerals "Hardness" and wondered what on Earth they were talking about, we have you covered!

 

 

What is a Minerals Hardness?

Hardness of a mineral refers to its resistance to scratching or abrasion. How well a mineral resists scratching is an important identification tool for geologists and field collectors. It is measured on a numerical scale from 1 (softest) to 10 (hardest) and is an extremely useful tool in mineral identification.

What is the Mohs Hardness Scale?

The Mohs Hardness Scale was created by a mineralogist named Frederich Mohs, who in 1812 developed the numerical scale of hardness using ten standard minerals and arranging them on a scale of 1 to 10.

Each mineral on the scale is slightly harder than the preceding one. Meaning a mineral with a higher number on the Mohs Hardness Scale can scratch any mineral with the same, or lower hardness value.

A mineral with a hardness of 9 can scratch a mineral with a hardness value of 8, a mineral with a hardness value of 8 can scratch a mineral with a hardness value of 7, and so on.

What Minerals are on the Mohs Hardness Scale?

The minerals chosen by Mohs for the scale are those which are fairly common and readily available to an average collector, all except the hardest mineral on the scale, Diamond (10). 

The table below lists the ten minerals on Mohs Hardness Scale, including their corresponding hardness levels. The scale goes from 1 being the softest, to 10 being the hardest.

It's important to note that the Mohs scale is what's called an "ordinal" scale, meaning that while there is an orderly value to the scale (1, 2, 3, 4 and so on), the distances between each numerical value varies. For example, while Diamond (10) is listed one numerical value above Corundum (9), Diamond is nearly 4 times harder than Corundum. And Corundum (9) is only twice as hard as Topaz (8).

Despite the ordinal nature of the scale, the Mohs hardness test remains a valuable tool for field collectors and geologists looking for quick mineral identification.

How Do You Measure a Minerals Hardness?

Every mineral has a corresponding hardness value between 1 and 10. A mineral can scratch another mineral with a lower value of hardness. For example, a Calcite specimen with a hardness value of 3 will easily be able to scratch a Gypsum specimen (hardness value of 2), because the Gypsum has a lower (softer) hardness value.

Below you will see a series of photos showing a Gypsum or Selenite crystal being scratched by a Calcite crystal.

Using the same scale, a mineral such as Fluorite (hardness of 4) could be scratched by a mineral with a higher hardness value, such as Ruby / Corundum with a hardness of 9.

What is a Mohs Hardness Test Kit?

Hardness test kits are small samples of minerals with corresponding hardness values present on the Mohs Hardness Scale. These kits can be found online and at most rock shops.

 

 

How Do You Field Test a Minerals Hardness?

If you don't have access to a hardness kit, many common items, or items you may already have in your hiking backpack while out rockhounding can serve as crude replacements. 

Even your hands can be used as test materials! Your fingernails have a hardness value of 2.5, a penny has a hardness value of 3, most knives have a hardness of 5.5 to 6, a steel nail has a hardness value of 7. So many common objects can also be used to test a minerals hardness.

For example, the muscovite Mica specimen seen below can easily be scratched with a fingernail.

 

What is the Best Way to Test a Minerals Hardness?

To test a minerals hardness, it is best to find a fresh, firm surface on the back of, or underneath a mineral specimen. By testing on the back or underside of a specimen, you ensure you aren't scratching or damaging the display-side of your specimen. 

Holding your specimen firmly, attempt to scratch it with the highest value sample on the test kit (Ruby / Corundum, hardness value 9), and work your way down the hardness scale. It is best to test on a small area for the higher hardness value minerals.

You should feel your specimen "catch" or see the testing sample slightly dig into your specimen. If a scratch is present, you know your specimen is softer, or has a lower hardness value than your testing sample.

Below is a series of photos showing a Calcite crystal specimen being scratched by a Ruby / Corundum specimen. The Ruby (hardness value of 9) easily scratches the Calcite (hardness value of 3).

If you feel your test sample slide across your specimen, and your specimen does not scratch, that means it is harder than the testing sample, and has a higher hardness value.

If it is not immediately obvious, test the specimen several times, and in varying directions until you can determine with a hand lens or jewelers loupe whether or not your specimen has been scratched. 

It is possible some minerals may have a hardness between whole number values. If a specimen can be scratched by all the hardness points down to Quartz (hardness 7), but not by Feldspar (hardness 6), that means it has a hardness value between 6 and 7, so 6.5.

Softness vs. Brittle

When testing a mineral specimen for hardness, make sure to test on a fresh surface, not one that is brittle or crumbly. Minerals that are fibrous, like some specimens of Malachite, may appear to be softer than they really are because of the nature of the crystals. Always make sure to use a firm, fresh surface to test the hardness.

The specimen of Gypsum and Fluorite shown below may be too crumbly to get an accurate hardness test.

 

Why is a Minerals Hardness Important?

While color and crystal shape are important identifying factors for minerals, the hardness is an equally important feature to ensure accurate mineral identification. 

It is also important for those shopping for Jewelry! Gemstones are minerals that are often cut and polished into faceted stones or cabochons to use in jewelry such as rings, earrings and pendants. A softer mineral like Fluorite (hardness of 4), may be easy to facet, but can easily be scratched by harder surfaces. Diamonds on the other hand, with a hardness of 10, are extremely scratch resistant, and often favored for engagement rings due to their durability and shine.

Conclusion

Hardness is both an important factor in determining a minerals identity, but also in choosing durable gemstones for jewelry. It is a useful tool for geologists, rockhounds and field collectors to help identify different rocks and minerals.