The Mohs hardness scale is a qualitative test that measures the hardness of a mineral by its ability to visibly scratch softer minerals. The scale isn’t perfect, but it’s a great tool for quick identification of rocks in the field. Here is a look at the scale, how ordinary minerals and objects rank, and how to perform the Mohs test.
Key Takeaways: Mohs Scale
- The Mohs hardness scale ranks minerals from 1 (softest) to 10 (hardest) based on scratch resistance.
- It is an ordinal scale, meaning the steps between numbers are not equal.
- Diamond (10) is much harder than corundum (9), not just one unit harder.
- The test compares an unknown material to objects of known hardness.
- Hardness is not the same as toughness, a hard mineral may still break easily.
- The Mohs scale is widely used for field identification, education, and material comparison.
The Mohs Hardness Scale for Minerals
Minerals are chemically pure solids that occur in nature. The Mohs scale ranks minerals on a scale from 1 to 10, in which 1 is the softest mineral and 10 is the hardest mineral. Diamond is the hardest mineral, with a Mohs hardness of 10. Talc is the softest mineral, with a hardness of 1.
The Mohs scale is ordinal, meaning the differences between numbers are not uniform. Each step represents a relative ranking rather than a fixed increase in hardness. For example, diamond (10) is four times harder than corundum (9), while corundum is twice as hard as topaz (8).
| Mohs Hardness | Mineral | Chemical Formula | Absolute Hardness |
|---|---|---|---|
| 10 | Diamond | C | 1500 |
| 9 | Corundum | Al2O3 | 400 |
| 8 | Topaz | Al2SiO4(OH−,F−)2 | 200 |
| 7 | Quartz | SiO2 | 100 |
| 6 | Orthoclase feldspar | KAlSi3O8 | 72 |
| 5 | Apatite | Ca5(PO4)3(OH−,Cl−,F−) | 48 |
| 4 | Fluorite | CaF2 | 21 |
| 3 | Calcite | CaCO3 | 14 |
| 2 | Gypsum | CaSO4·2H2O | 2 |
| 1 | Talc | Mg3Si4O10(OH)2 | 1 |
Why the Mohs Scale Is Not Linear
Although the Mohs scale runs from 1 to 10, it does not increase evenly. Instead, hardness increases exponentially. For example:
- Corundum (9) is about 4 times harder than topaz (8)
- Diamond (10) is about 4 times harder than corundum (9)
- Diamond is roughly 1500 times harder than talc (1)
This means the difference between 9 and 10 is far greater than the difference between 2 and 3.
Mohs Hardness Scale of Common Materials
Where, exactly, a specimen falls on the Mohs scale it not determined solely by its chemical formula. Minerals may have multiple crystals structures and usually contain additional elements. But, this table is a good comparison of common values.
| Mohs Hardness | Materials |
|---|---|
| 0.2–0.3 | cesium, rubidium |
| 0.5–0.6 | lithium, sodium, potassium, candle wax |
| 1 | talc |
| 1.5 | gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice |
| 2 | calcium, selenium, cadmium, sulfur, tellurium, bismuth, gypsum |
| 2-2.5 | fingernail, mica, rock salt (halite), amber |
| 2.5-3 | gold, silver, zinc, lanthanum, aluminum, cerium, jet, pearl |
| 3 | calcite, copper, chalk, dentin, thorium, arsenic, antimony |
| 3.5 | platinum, sphalerite, chalcopyrite |
| 4 | fluorite, nickel, iron |
| 4-4.5 | ordinary steel |
| 5 | apatite, tooth enamel, obsidian, palladium, zirconium |
| 5.5 | glass, beryllium, cobalt, hafnium, molybdenum, turquoise, lapis lazuli, opal |
| 6 | orthoclase, uranium, titanium, germanium, niobium, manganese |
| 6-7 | iron pyrite, silicon, ruthenium, opal, peridot, tanzanite, iridium, jade, rhodium, pyrite, garnet, jasper, chrysoprase, chalcedony |
| 7 | quartz, citrine, amethyst, osmium, rhenium, vanadium, unglazed porcelain, zircon |
| 7.5-8 | beryl, emerald, zircon, spinel, tungsten |
| 8 | topaz, cubic zirconia, spinel, hardened steel |
| 8.5 | chrysoberyl, silicon nitride, tantalum carbide, chromium |
| 9 | rubies and sapphires (corundum), tungsten carbide, aluminum oxide (alumina), titanium nitride |
| 9-9.5 | moissanite (silicon carbide or carborundum), boron carbide, zirconium carbide |
| 9.5- almost 10 | boron nitride, boron, boron carbide, stishovite, titanium diboride |
| 10 | diamond, carbonado (black diamond) |
From the table, note which pure elements are soft and which are hard. Metals that becomes liquids near room temperature are the softest. Nonmetals are often softer than metals, particularly transition metals. While diamond is pure carbon and is a nonmetal, other carbon allotropes are softer.
How to Interpret Scratch Results
A scratch test does not measure how deep a scratch is, only whether one material can produce a visible mark on another.
- If A scratches B → A is harder than B
- If A does not scratch B → A is softer than or equal to B
- Materials of equal hardness may scratch each other
To improve accuracy, test multiple points and compare against several reference materials.
Perform the Mohs Hardness Test
The Mohs hardness test is simple and requires only a set of reference materials with known hardness values. You compare an unknown sample against these materials to determine its approximate hardness.
Handy materials include:
- your fingernails (2.5)
- a copper coin or piece of chalk (3.5)
- a pocket knife or piece of glass (4.5-5.5)
- a quartz point or unglaze porcelain (7.0)
- a hardened drill bit (8.5)
Once you have a collection of materials of representative hardness values, perform the test:
- Choose a fresh, clean surface on the material you are testing.
- See if you can scratch it with the point of an object with known hardness. This object is your “tool.” If you feel it “bite” into the surface of the test material, it’s probably harder than your sample.
- Inspect your sample for a scratch or etched line. Turn it under good lighting so a scratch is more visible. Run your fingernail across the sample and see if you feel a scratch.
- If the tool scratches the sample, then the sample has a lower Mohs hardness value than the tool. For example, if you scratch a sample with a quartz point, then the sample has a hardness less than 7.
- If the tool does not scratch the sample, then it is harder than the tool. This is because materials of equal hardness scratch one another. For example, if you cannot scratch a sample with a quartz point, its hardness is greater than 7.
Use other tools you have available to narrow down the possible hardness value. For example, if quartz scratches the sample, you know it is harder than 7. See if a hardened drill bit scratches it. If it does not, then the hardness is between 7 and 8.5. If no scratch appears, the samples is harder than 8.5.
Common Everyday Reference Objects
These familiar items help estimate hardness in the field:
| Object | Approximate Mohs Hardness |
|---|---|
| Fingernail | 2.5 |
| Copper coin | 3–3.5 |
| Knife blade | 5–5.5 |
| Window glass | 5.5 |
| Steel file | 6.5 |
| Quartz | 7 |
These references are especially useful when a full mineral kit is not available.
Mohs Scale Uses
The Mohs hardness scale has practical applications in geology, materials science, and industry:
- Mineral identification in the field and classroom
- Gemstone evaluation and comparison
- Material selection for tools and abrasives
- Engineering applications, including coatings and wear resistance
- Electronics, such as evaluating glass for screens and displays
While simple, the scale provides a fast and useful way to compare materials.
History
German mineralogist Friedrich Mohs introduced the Mohs hardness scale in 1822 in his Treatise on Mineralogy. He designed the scale as a practical tool for identifying minerals using simple scratch tests.
The concept of comparing hardness by scratching is much older. The Greek philosopher Theophrastus described similar methods around 300 BC in On Stones. Mohs refined these ideas into a standardized ranking system that remains widely used today.
See the appearance of the birthstone minerals before they get cut into gemstones and see their chemical formulas and Mohs hardness values.
Limitations of the Mohs Hardness Scale
Limitations of the Mohs scale include its lack of precision and that fact that it is a potentially destructive test. For example, if you test a gemstone by seeing whether or not corundum scratches it, any mineral softer than 9 sustains a permanent mark.
- It is qualitative, not precise
- The scale is non-linear, so differences between values vary greatly
- Results depend on surface condition and testing technique
- It may damage valuable samples, such as gemstones
- It does not measure other properties like toughness or strength
Because of these limitations, scientists often use more precise hardness tests in laboratories.
Alternatives to the Mohs Test
More precise hardness measurements use controlled indentation rather than scratching:
- Vickers hardness test: Uses a diamond pyramid indenter
- Rockwell hardness test: Measures depth of indentation
- Brinell hardness test: Uses a hardened steel or carbide ball
- Knoop hardness test: Designed for brittle materials and thin samples
These methods provide numerical values that allow direct comparison between materials.
Comparing Hardness Scales
| Scale | Type | What It Measures |
|---|---|---|
| Mohs | Scratch | Relative hardness |
| Vickers | Indentation | Absolute hardness |
| Rockwell | Indentation | Depth under load |
| Brinell | Indentation | Resistance to deformation |
Hardness vs Toughness vs Strength
Hardness describes resistance to scratching, but it is only one of several mechanical properties:
- Hardness: Resistance to scratching (Mohs scale)
- Toughness: Resistance to breaking or fracturing
- Strength: Ability to withstand force without deforming
For example, diamond is extremely hard but relatively brittle, while jade is less hard but much tougher and resistant to breaking.
FAQs
Is diamond the hardest material?
Diamond is the hardest natural mineral on the Mohs scale, but some synthetic materials are harder.
Can two materials of the same hardness scratch each other?
Yes. Materials with equal hardness can scratch one another.
Does hardness mean a material won’t break?
No. Hard materials can still be brittle and break easily.
Why is the Mohs scale still used?
It is simple, fast, inexpensive, and useful for field identification.
References
- Mohs, Friedrich (1825) [Original German publication in 1822]. Treatise on Mineralogy: Or, The Natural History of the Mineral Kingdom. Translated by Haidinger, William. United Kingdom: A. Constable and Company.
- Mukherjee, Swapna (2012). Applied Mineralogy: Applications in Industry and Environment. Springer Science & Business Media. ISBN 978-94-007-1162-4.
- Smith, R.L.; Sandland, G.E. (1922). “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: 623–641.
- Zhang, P. (September 2011). “General relationship between strength and hardness”. Materials Science and Engineering A. 529: 62. doi:10.1016/j.msea.2011.08.061