Mohs Hardness Scale
A rapid hardness test for field and classroom use
Article by: Hobart M. King, Ph.D., RPG
What is Mohs Hardness Scale?
One of the most important tests for identifying mineral specimens is the Mohs Hardness Test. This test compares the resistance of a mineral to being scratched by ten reference minerals known as the Mohs Hardness Scale (see table at left). The test is useful because most specimens of a given mineral are very close to the same hardness. This makes hardness a reliable diagnostic property for most minerals.
Friedrich Mohs, a German mineralogist, developed the scale in 1812. He selected ten minerals of distinctly different hardness that ranged from a very soft mineral (talc) to a very hard mineral (diamond). With the exception of diamond, the minerals are all relatively common and easy or inexpensive to obtain.
Making Hardness Comparisons
"Hardness" is the resistance of a material to being scratched. The test is conducted by placing a sharp point of one specimen on an unmarked surface of another specimen and attempting to produce a scratch. Here are the four situations that you might observe when comparing the hardness of two specimens:
- If Specimen A can scratch Specimen B, then Specimen A is harder than Specimen B.
- If Specimen A does not scratch Specimen B, then Specimen B is harder than Specimen A.
- If the two specimens are equal in hardness then they will be relatively ineffective at scratching one another. Small scratches might be produced, or it might be difficult to determine if a scratch was produced.
- If Specimen A can be scratched by Specimen B but it cannot be scratched by Specimen C, then the hardness of Specimen A is between the hardness of Specimen B and Specimen C.
Mohs Hardness Testing Procedure
- Begin by locating a smooth, unscratched surface for testing.
- With one hand, hold the specimen of unknown hardness firmly against a table top so that the surface to be tested is exposed and accessible. The table top supports the specimen and helps you hold it motionless for the test.
- Hold one of the standard hardness specimens in the other hand and place a point of that specimen against the selected flat surface of the unknown specimen.
- Firmly press the point of the standard specimen against the unknown specimen, and firmly drag the point of the standard specimen across the surface of the unknown specimen.
- Examine the surface of the unknown specimen. With a finger, brush away any mineral fragments or powder that was produced. Did the test produce a scratch? Be careful not to confuse mineral powder or residue with a scratch. A scratch will be a distinct groove cut in the mineral surface, not a mark on the surface that wipes away.
- Conduct the test a second time to confirm your results.
Mohs Hardness of Common Minerals
Mohs Hardness Testing Tips
- A list of minerals in order of hardness can be a handy reference. If you determine that a specimen has a hardness of Mohs 4, you can quickly get a list of potential minerals.
- Practice and experience will improve your abilities when doing this test. You will become faster and more confident.
- If the hardness of the unknown specimen is about 5 or less, you should be able to produce a scratch without much exertion. However, if the unknown specimen has a hardness of about 6 or greater, then producing a scratch will require some force. For those specimens, hold the unknown firmly against the table, place the standard specimen against it, press firmly with determination, then holding pressure slowly drag the standard specimen across the surface of the unknown.
- Don't be fooled by a soft standard specimen producing a mark on a hard unknown. That mark is like what a piece of chalk produces on a blackboard. It will wipe off without leaving a scratch. Wipe your finger across the tested surface. If a scratch was produced, there will be a visible groove. If marks wipe away then a scratch was not produced.
- Some hard materials are also very brittle. If one of your specimens is breaking or crumbling rather than scratching, you will have to be very careful while conducting the test. Testing tiny or granular specimens can be difficult.
- Some specimens contain impurities. If the results of your test are not visibly conclusive, or if the information from your test does not conform with other properties, do not hesitate to do the test again. It is possible that a small piece of quartz (or another impurity) was embedded in one of your specimens.
- Don't be wimpy! This is a very common problem. Some people casually rub one specimen back and forth against another and then look for a mark. That is not how the test is done. It is done with a single, determined motion with the goal of cutting a scratch.
- Be careful. When you hold the unknown specimen against the table, position it so that the known specimen will not be pulled across one of your fingers.
- This test should be done on a lab table or work bench with a durable surface or a protective covering. Don't do this type of testing on fine furniture.
- Test tiny particles or grains by placing them between two pieces of an index mineral and scraping them together. If the grains are harder than the index mineral, scratches will be produced. If the grains are softer they will smear.
Hardness of Common Objects
Some people use a few common objects for quick hardness tests. For example, a geologist in the field might always carry a pocket knife. The knife can be used for a quick hardness test to determine if a specimen is harder or softer than Mohs 5 to 6.5.
Before using these objects as quick testing tools, it is a good idea to confirm their hardness. Some knives have harder steel than others. Test yours and then you know its hardness.
These common objects can also be useful if you don't have a set of reference minerals. We included quartz in this list because it is a ubiquitous mineral. In the field you are often no more than a few steps away from a piece of quartz.
An alternative to using the reference minerals for testing is a set of "hardness picks." These picks have sharp metal points that you can use for very accurate testing. The picks allow much more control, and their sharp points can be used to test small mineral grains in a rock.
The sharp picks can be used easily and either produce a scratch if they are harder than the specimen being tested or leave behind a tiny streak of metal if they are softer. Examine the test site with a hand lens to see the results of your test.
We have used hardness picks and think that they do a great job. They are easier to use and more accurate than testing with specimens. They can be resharpened when they dull. The only downside is their price (about $80 per set).
Harder than Diamond, Softer than Talc?
Diamond is not the hardest substance known, but the materials that are harder are much more rare. Researchers have reported that wurtzite boron nitride and lonsdaleite can be harder than diamond. 
It is unlikely that you will find a mineral that is softer than talc. However, a few metals are softer. These include: cesium, rubidium, lithium, sodium, and potassium. You will probably never need to test their hardness. 
Mohs Scale of Hardness Compared to Others
When Friedrich Mohs developed his hardness scale in 1812, very little information about mineral hardness was available. He simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10. It was a relative scale in which a mineral of unknown hardness could be tested against a group of ten index minerals to see where it positioned on the scale.
The Mohs scale has stood the test of time and has been widely used throughout the world for over 200 years - mainly because it is easy-to-do, inexpensive and people quickly understand it. Other hardness tests have been devised but none of them are in as widespread use.
A “Mohs hardness” is a relative integer-scale comparison of “resistance to being scratched.” Most other hardness scales use “resistance to indentation under a stylus to which a specific amount of pressure is applied for a specific length of time.” Although these tests differ from Mohs hardness in their procedure, they are all tests of the resistance to atoms being dislodged from their positions by pressure against the surface of a mineral specimen.
One of these scales is the Vickers Hardness Scale. In the Vickers test, the size of the indentation is microscopically estimated and used to calculate a hardness value. The Vickers hardness values form a continuous scale which provides more information about the hardness of minerals when compared to the integer values of the Mohs scale. A table comparing the Mohs scale minerals to their Vickers hardness is shown here along with a graph of the data. The graph shows that in terms of Vickers hardness, the gaps between the integer values of the Mohs scale are not uniform in width. In addition the gaps between minerals of higher Mohs hardness are much broader than those between the softer minerals. In terms of Vickers hardness, diamond is enormously harder than corundum.
Hardness Variations in a Single Mineral
Although reference books and websites often list a single hardness for each mineral, many minerals have variable hardness. They have greater or lesser hardness depending upon the direction in which they are being scratched.
A well-known example of a mineral with variable hardness is kyanite. Kyanite frequently occurs in blade-shaped crystals. These crystals have a hardness of about 5 if they are tested parallel to the long axis of the crystal, and a hardness of about 7 if they are tested parallel to the short axis of a crystal. Why? These different directions encounter different bonding environments in the kyanite crystal. The bonds that resist scratching parallel to the long axis of the bladed crystal are weaker than those encountered when scratching across the width of the crystal. Intermediate hardnesses are encountered in other directions.
Another example is diamond. The people who cut diamonds have known about its variable hardness for hundreds of years. They know that parallel to the octahedral crystal faces, a diamond crystal is almost impossible to saw and very difficult to polish. The diamond can be broken in this direction by cleaving, and the best method for cutting it in this direction is with a laser. The softest and best direction to saw or polish a diamond crystal is parallel to its cubic crystal faces. This information is critical knowledge for the craftsmen who plan the design of a faceted diamond. Understanding it and working with it saves time, saves money and creates a better product with less waste.
Weathering can also influence the hardness of a mineral specimen. Weathering changes a mineral's composition, with the weathering product usually softer than the original material. When testing the hardness or streak or other property of a mineral, the best way to test is on a freshly broken surface with expected luster that has not been exposed to weathering.
About Hardness Tests
The hardness test developed by Friedrich Mohs was the first known test to assess resistance of a material to scratching. It is a very simple but inexact comparative test. Perhaps its simplicity has enabled it to become the most widely used hardness test.
Since the Mohs Scale was developed in 1812, many different hardness tests have been invented. These include tests by Brinell, Knoop, Rockwell, Shore and Vickers. Each of these tests uses a tiny "indenter" that is applied to the material being tested with a carefully measured amount of force. Then the size or the depth of the indentation and the amount of force are used to calculate a hardness value.
Because each of these tests uses a different apparatus and different calculations, they can not be directly compared to one another. So if the Knoop hardness test was done, the number is usually reported as a "Knoop hardness." For this reason, Mohs hardness test results should also be reported as a "Mohs hardness."
Why are there so many different hardness tests? The type of test used is determined by the size, shape, and other characteristics of the specimens being tested. Although these tests are quite different from the Mohs test, there is some correlation between them. 
Hardness, Toughness, and Strength
When testing for hardness, remember that you are testing "the resistance to scratching." During the test, some materials might fail in other ways. They could break, deform, or crumble instead of scratching. Hard materials often break when subjected to stress. This is a lack of toughness. Other materials might deform or crumble when subjected to stress. These materials lack strength. Always keep in mind that you are testing for the resistance to being scratched. Don't be fooled by other types of failure in the specimen being tested.
Uses for Hardness Tests
The Mohs Hardness Test is almost exclusively used to determine the relative hardness of mineral specimens. This is done as part of a mineral identification procedure in the field, in a classroom, or in a laboratory when easily identified specimens are being examined or where more sophisticated tests are not available.
In industry, other hardness tests are done to determine the suitability of a material for a specific industrial process or a specific end-use application. Hardness testing is also done in manufacturing processes to confirm that hardening treatments such as annealing, tempering, work hardening, or case hardening have been done to specification.
 Scientists Discover Material Harder Than Diamond - Lisa Zyga, website article on Phys.org, February 2009.|
 Mohs Scale of Mineral Hardness: Wikipedia article, last accessed July 2016.
 Material Hardness: website article, Center for Advanced Life Cycle Engineering, University of Maryland, last accessed July 2016.
Some Notes on Spelling
Mohs Hardness Scale is named after its inventor, Friedrich Mohs. This means that an apostrophe is not needed when typing the name of the test. "Moh's" and "Mohs' " are incorrect.
Google is really smart about these names. You can even type "Moe's Hardness Scale" as a query and Google knows to return results for "Mohs Hardness Scale." :-)
|Diamonds Do Not Form From Coal|
Find Other Topics on Geology.com: