What is the Mohs Hardness Scale? Why Is it important, and how does it work? We have an insight into all these, including how to do a Mohs hardness test, Mohs hardness test kits, and much more. However, Before we go any further, you deserve to know that the correct spelling is the Mohs Hardness Scale and not Moh's or Mohs' (i.e., one with an apostrophe). The hardness scale is Mohs, named after the inventor Carl Friedrich Christian Mohs. So, an apostrophe is unnecessary.
What is hardness?
Hardness or softness is the ability of a material to resist abrasions (scratching) or indentation, i.e., the ability to withstand localized permanent deformation from indentation or abrasion. In other words, hardness measures the ease or difficulty of creating microfractures or displacing atoms. This displacement or microfracture happens when bonds break.
Different materials have different hardness depending on their bond strength, mineral lattice structure, and other factors. For instance, both graphite and diamonds are made of carbon. However, bonds and mineral lattice dramatically change their hardness.
So, both resistance to indentation and scratching measure hardness. However, Mohs hardness tests relative resistance to abrasion of unknown minerals, gemstones, or materials against a standard ten-mineral scale, 1 to 10.
What is the Mohs Hardness Scale, and does it work?
The Mohs Hardness Scale is a simple and practical way to identify minerals, gemstones, and other objects in the field by considering their hardness. This scale ranks minerals on a ten-point scale, i.e., numbers 1 to 10. Each of the numbers relative hardness (resistance to scratching or abrasion) of 10 standard minerals, from the softest or lowest, i.e., 1, represented by talc, to the hardest, represented by diamond at 10.
This scale works on simple logic, i.e., a mineral (object or material) that scratches the other is harder and ranks higher and vice versa. However, if two minerals ineffectively scratch each other, they have comparable or equal hardness.
Therefore, you can use it to find the hardness of unknown minerals (specimen) by testing how it resist scratching using these ten minerals since their scale has their hardness. But there are picks and familiar objects you can use at home to test hardness.
One thing I must emphasize is what a scratch means. When we say a scratch in the Mohs scale, it refers to creating a non-elastic or permanent dislocation that you can see by the naked eye. It doesn’t refer to microscopic dislocations commonly seen in harder materials, even if they are permanent or affect the specimen structurally, i.e., their structural integrity.
Lastly, the Mohs Hardness Scale was created by Carl Friedrich Christian Mohs (1773-1893), a German chemist and mineralogist, in 1812. Here are the original minerals he considered:
| Material |
Mohs hardness |
Description and examples |
| Diamond |
10 |
Diamond is pure carbon whose atoms have a diamond cubic structure. Its color ranges between white and brown or yellow. Diamond is the hardest mineral that scratches corundum, and people use it as a glass cutter. |
| Corundum |
9 |
Corundum is a naturally occurring, very hard aluminum oxide mineral. Its pure form is what vendors sell as sapphire and ruby gemstones. It is available in all rainbow colors, and it can cut glass. |
| Topaz |
8 |
Topaz is an aluminum and fluorine silicate sold as gemstones. It is mainly gray, pale yellow, or brown. However, other colors like violet, pink, or light green exist. Topaz can scratch glass very easily. |
| Quartz |
7 |
Quartz is a white to clear, hard crystalline mineral made of silica or silicon dioxide. This mineral will scratch glass with ease. |
| Orthoclase |
6 |
Orthoclase (potassium aluminosilicate) is an abundant type of alkali feldspar rock-forming mineral. It is often colorless, greyish-yellow, greenish, white, or pink. A knife cannot scratch. However, glass will scratch it, but with difficulty. |
| Apatite |
5 |
Apatite refers to any calcium phosphates that make up a larger percentage of phosphate rock, bones, and teeth. Its colors vary. So, it may be colorless, white, blue, green, brown, purple, gray, pink, yellow, etc. A knife will scratch it with difficulty, but not fluorite. |
| Fluorite |
4 |
Fluorite or fluorspar is a calcium fluoride mineral that a knife easily scratches, but not a penny. It is often colorless. However, impurities make it deeply colored, i.e., golden-yellow, green, pink, blue, brown, lilac, purple, or champagne. |
| Calcite |
3 |
Calcite is a stable calcium carbonate polymorph abundant in limestone. It is colorless to white in its pure form. However, impurities may make it reddish, pinkish, greenish, bluish, brown, black, yellow, or lavender. This mineral is very easy to scratch using a knife, but a penny barely scratches it. |
| Gypsum |
2 |
Gypsum is a transparent, colorless, grayish, mottled-white, soft, hydrous sedimentary rock made of calcium sulfide. Your fingernail can scratch it, but not as easily as talk.
|
| Talc |
1 |
Talc (talcum), a hydrated magnesium silicate, is a whitish-grey to green, transparent to translucent mineral. It is very easy to scratch using your fingernail and feels greasy. |
Mohs hardness scale box with each of the ten minerals – Photo credit: Hannes Grobe, Wikimedia, CC BY 3.0
How do you practically use this scale? We will take an example of an unknown specimen A. If the hardest mineral A scratches apatite (H=5), but orthoclase (H=6) scratches it, the hardness of specimen A is between 5 and 6.
For students who want to memorize the Mohs Hardness Scale, a simple mnemonic will be The Geologist Can Find An Ordinary Quartz (that) Tourists Call Diamond. Each of the first or bolded letter represent a mineral on the scale. It should help you remember Mohs scale minerals.
Minerals and rocks are sometimes categorized as soft, medium, hard, and hardest. Here is the corresponding Mohs hardness.
| Term |
Mohs Hardness |
Description or Examples |
| Soft |
1-2 |
Your fingernails can scratch it. Examples are talc and gypsum |
| Medium |
3-5 |
A knife or nail will scratch it |
| Hard |
6-9 |
A knife doesn’t scratch it, and it can scratch glass |
| Hardest |
10 |
Can scratch any other material |
1. What do sources around the web say Mohs Hardness Scale is?
Here is how other sources define the Mohs Hardness Scale:
The Mohs scale of mineral hardness (/moʊz/) is a qualitative ordinal scale, from 1 to 10, characterizing scratch resistance of various minerals through the ability of harder material to scratch softer material. - Wikipedia
Mohs hardness is a rough measure of the resistance of a smooth surface to scratching or abrasion, expressed in terms of a scale devised (1812) by the German mineralogist Friedrich Mohs. - Britannica
Mohs’ hardness measures the relative hardness and resistance to scratching between minerals. - Mineral Society of America
The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. - New World Encyclopedia
To conclude, it is correct to say that the Mohs Hardness Scale ranks material according to the easy or difficulty of getting scratches. All materials fall between 1 and 10, the hardest at the top and softest at the bottom.
2. Why is the Mohs hardness scale important?
The first reason is that it helps you identify and grade minerals and gemstones, considering their hardness. Also, geology students can use Mohs Hardness Tests to determine various minerals in a lab, classroom, or field study. However, for a conclusive result, you need to consider other mineral or rock properties like:
- Luster
- Color
- Streak
- Specific gravity
- Cleavage
- Fracture
- Tenacity
- Diaphaneity
- Crystal shape
- Acid reaction
- Magnetism
Secondly, hardness is related to durability: the harder, the more durable. Manufacturers use Mohs Hardness Tests to determine how resilient some components, like glass on the LCD or OLEDs encapsulation, or even gemstones. Also, they use it to measure how well hardening treatments like tempering or annealing perform. This way, they can tell if they meet specified requirements.
Thirdly, mineral hardness can guide you on where to use it – from milling, cutting, drawing dies, abrasive materials, etc. For instance, a diamond makes cutting tools for metals, forming dies, and drilling bits, while soft talc works as a bath powder.
Did you know that durability has an impact on price? The more durable a gem, metal, or mineral is, the more likely it will cost you more. So, this Mohs hardness scale should give you a hint on price. Sapphires, diamonds, and ruby are expensive and are on the harder and more durable side.
Lastly, A scratch test can show the relative hardness of two unknown specimens by considering how easy or hard it is to scratch them. A softer one will require less effort and a hard one more. For instance, quartz will scratch calcite more easily than orthoclase.
Mohs Hardness test kits, picks, and familiar objects
To conduct these tests, you need minerals or things (kits) whose Mohs hardness scale you know and your specimen. The specimen is the rock or mineral you are trying to find its relative hardness.
For kits, you can use the Mohs hardness test kit, picks, or familiar objects whose hardness you know. Let us look at each of these test kits.
1. Mohs hardness test kit
The Mohs hardness test kit is a scratch-testing kit with all the minerals in the Mohs Hardness Scale except diamond. Why? Diamond is expensive and needs mounting on a handle since it’s small.
You can use this kit to determine the relative hardness of most minerals and rocks. However, this kit may not be accurate, and some small mineral inclusion in the specimen may skew results. Nonetheless, it is cheaper and more reliable.
2. Mohs Hardness Scale picks set
Mohs hardness pick is a more accurate mineral or rock scratch-testing kit. It often comes with double-ended color-coded stylus picks with labeled (engraved) sharp points or tips corresponding to Mohs Hardness Scale 2 to 9 and a 3.5 and 5.5 hardness plate. Also, you will get a 100-grit sharpening or grinding stone.
Some kits may have a magnet and streak plate. Also, they come with hardness testing instructions, a paper table, and a holding case.
To use them, hold your specimen (unknown) and scratch or drag their pointed edges (pick ends) over it to see if it scratches it (creates a groove) or not. Those softer than the specimen will leave a metallic streak behind, while harder ones will scratch (groove)on the unknown mineral or rock. If their tips get dull, use the grinding stone to sharpen them.
One advantage of the hardness picks is that they give you more control. Also, thanks to their pointed ends, you can use these tools to test even mineral grains in a rock or a small mineral.
Lastly, if you want to buy your Mohs hardness picks, Mineralab has an excellent selection on Amazon.com, including a Deluxe Hardness Pick. These Mohs hardness test kits range from 100 to $150 depending on what they come with and where you buy them.
3. Use of objects of known Mohs hardness – homemade kit
If you don’t want to buy a Mohs hardness test kit, you can improvise one. It requires using commonly available objects or materials whose hardness is known on the Mohs scale. Here is a list of these materials:
| Material or Object |
Mohs Hardness |
| Fingernails |
2.5 |
| Silver or gold |
2.5-3 |
| Copper penny |
3 |
| Nail |
4 |
| Knife blade |
5.5 |
| Glass |
5.5 |
| Hardened steel file |
6.5 |
| Streak plate or unglazed porcelain |
6.5-7 |
| Quartz |
7 |
| Masonry drill |
8.5 |
| Silicon carbide blade |
9.5 |
Common objects and their Mohs hardness
You will use the above scale just like the Mohs hardness test kit. For instance, a copper penny (H=3) cannot scratch a specimen, but a knife (H = 5.5) does; its hardness lies between 3 and 5, i.e., it’s harder than calcite but softer than orthoclase.
One downside of these objects is that their hardness varies and may give you inaccurate results. Lab tests have proven this fact.
How to do a Mohs hardness test?
From its definition, Mohs hardness refers to a mineral’s relative resistance to abrasion or scratching determined by scratching it with a material or object whose hardness you know on the Mohs Hardness Scale.
So, at this point, we assume you have your Mohs hardness test picks, kits, or familiar objects whose hardness you know.
Your work will be to find the softest material or object that scratches the unknown specimen (sample) and the hardest that the specimen scratches. The hardness will then be between these two.
However, before you start, we insist you conduct your test on a workbench or lab table with a protective and durable cover. Please don’t do it on a fragile or unprotected furniture surface. Place cardboard, rubber pad, or even PVC mat if you must use your fine furniture.
To help you conduct the exercise, search for a printable Mohs Hardness Scale if you haven’t memorized all the minerals on the chart. There are many available online.
Here are the steps to follow to find out the relative hardness of an unknown specimen:
- Select unmarked or unmarred unknown minerals or rocks, i.e., the one whose hardness you need to determine or what we will call a specimen.
- Place this unknown specimen on the table and hold it firmly against it (it shouldn’t move) with one hand, exposing a smooth surface, i.e., this surface should be accessible. To avoid injury, ensure your fingers are parallel to the direction you will scratch the specimen.
- Position a point or sharp corner of your mineral or object of known Mohs hardness on the smooth edge. Then press it firmly (apply pressure) and drag it across the smooth surface of the unknown specimen away from your body to avoid injuries. Do one scratch, i.e., don’t make back-and-forth scratches.
- Rub or brush off any dust (mineral fragments formed) and check for a scratch (groove) on the unknown specimen. You can also feel it with your fingernail. If there is a scratch, then the unknown sample is softer.
- If there is no scratch, reverse the test to see if the unknown specimen scratches your mineral or object of known Mohs hardness. If it does, it is harder.
- Repeat the test to check if the results will be consistent. You can also use another sample of the same mineral or rock.
To get relative hardness, you must conduct several tests with objects or minerals of different hardness. Start on the lower side and slowly go up until you find the softest that makes a scratch on your unknown specimen. Then, the relative hardness of your sample will be between the previous one that didn’t make a scratch and the one that did.
Alternatively, if you reverse your experiment, you can find the hardest material your unknown specimen scratches and the one it doesn’t. Relative hardness will be between these two.
However, if the hardness of the sample and object or mineral you are using have comparable hardness, you don’t have to do many tests. Just ensure you reverse the test to confirm that they ineffectively scratch each other.
Lastly, the test may dislodge loose grains by a fine-grained, crumbly, or powdery specimen. So, you are unlikely to get the correct hardness. Such materials hinder accurate hardness measurement. So, it’s not good for testing ceramics, steel, or other industrial materials.
How to make the Mohs hardness test experiment a success?
Besides the above steps, here are some tips that may ensure your experience is a success:
- When scratching, apply enough force. How much effort you use depends on the hardness of the specimen. Usually, minerals with hardness above 6 require more effort than those below. So, it would be best if you did some practice.
- Do the test on several parts of the unknown specimen to ensure your specimen is not just an inclusion. If it is a large sample, pick a homogenous spot.
- Do you have a very small specimen? Place it between two objects or minerals of known hardness for small scrape them together. If the two objects get scratched, it is softer than the specimen. And if not, i.e., it gets smeared, it is harder.
- For a valuable unknown specimen, you don’t want to damage, reverse-test it, i.e., use its corner or point for scratching the minerals in your test kit.
- Handle smaller or granular pieces with extra care to avoid injuries
The downside of the Mohs Hardness Scale or Test
We agree that the Mohs Hardness Scale is undoubtedly the easiest and most inexpensive method of determining mineral and material hardness. However, it has its downside.
Before we look at how to conduct the Mohs Hardness Test, you need to know a few misconceptions.
a). It measures resistance to scratching, i.e., ease or difficulty of getting scratched, not hardness, stiffness, or strength
Mohs hardness measures resistance to scratching on material when it encounters a sharp point. It doesn’t measure toughness that tells you how a mineral will resist breaking, chipping, impact, or crackling.
Similarly, Mohs hardness doesn’t tell you more about a mineral’s stiffness or strength. Strength measures include compressive, tensile, fatigue, or shear and tell you how a material resists pressure or force. This is especially true in ceramics and some metals.
Stiffness, on the other hand, tells you more about how a material returns to its original shape when you remove a bending.
Therefore, you cannot know mineral stiffness, toughness, or strength from the Mohs Hardness Scale. And this test alone cannot tell if a specific material is suitable for use in industrial settings. For instance, can you know if it will endure other requirements like strength and toughness that may be necessary?
I hope this clarifies the hardness vs. toughness vs. strength issue or misconception.
That is not all. Inclusions and fractures may affect a mineral’s durability even though it may rank higher on the Mohs Hardness Scale. A case worthwhile is some Emerald. So, other tests may be necessary to determine durability.
b). Values of the chart give relative hardness to another material, not actual
The hardness values, i.e., 1 to 10 on the Mohs Hardness Scale, represent relative strength, not exact or absolute. So, these values tell you which between two minerals or objects resist abrasion more or less – nothing about true hardness. And the spaces between the two may seem the same, but the hardness difference is not the same.
Therefore, the scale is ordinal and not proportional.
A good example is a diamond and corundum, which rank 10 and 9 on the Mohs Hardness Scale. However, if you compare their absolute strength, diamonds are more than three times stronger than corundum. So, it is not a linear scale.
See the chart below of Mohs hardness vs. absolute hardness
| Mohs Hardness |
Mineral |
Absolute Hardness |
| 10 |
Diamond |
1500 |
| 9 |
Corundum |
400 |
| 8 |
Topaz |
200 |
| 7 |
Quartz |
100 |
| 6 |
Feldspar |
72 |
| 5 |
Apatite |
48 |
| 4 |
Fluorite |
21 |
| 3 |
Calcite |
9 |
| 2 |
Gypsum |
2 |
| 1 |
Talc (talcum) |
1 |
The chart above gives you a comparison between Mohs hardness and absolute hardness.
c). Some mineral’s hardness varies with the direction of scratching
For some minerals, the hardness may depend on the direction of your scratch or crystal face. A good example is the kyanite mineral. Its hardness can be 5.5 or 7.0, depending on the axis of your scratch.
Therefore, it is good to test various crystal faces or directions before concluding.
4. More hardness tests – Vickers, Brinell, Rockwell, Shore, and Knoop
Hardness is the ability of a material to resist abrasions (scratching) or indentation. Mohs created his method by considering resistance to abrasion or scratching. When he devised the technique (1812), people knew little about the hardness of materials or minerals.
How did he do it? To create a hardness testing scale, Mohs selected ten minerals, from talc, the softest, to diamond, the hardest. He then gave each mineral an integer, 1 to 10, in the order of their increased hardness or resistance to scratches.
With this scale, known as the Mohs Hardness Scale, it was possible to test the relative hardness of other materials and rank them between 1 and 10. Unfortunately, this scale is ordinal, not proportional. So, it gives you relative hardness, not absolute.
However, 200 years later, the Mohs Hardness Test remains the most popular way to determine the hardness of minerals by geologists, especially in the field. It has stood the test of time because it is easy to understand, use, and inexpensive. All you need is the Mohs kit.
Besides the Mohs hardness test, there are other newer ways to test the hardness of materials. Popular ones are Vickers, Brinell, Rockwell, Shore, and Knoop. All these methods use an indention test to determine hardness. By measuring indention size and considering the force applied, it is possible to compute hardness. However, each method has its scale.
To give you an experience, let us examine Knoop, a microhardness test ideal for thin sheets and brittle to very brittle materials. The test involves making a small indentation on the test specimen us a known weight. Then, a measurement of the indentation is done using a microscope. Afterward, they can compute the hardness using the Knoop hardness HK formula. The resultant values are on a continuous scale and represent actual hardness.
Below is a graph of the Knoop and Mohs hardness scale. It should help understand how hard each of the ten minerals is.
From the above Knoop vs. Mohs hardness scale, you will notice that the Knoop scale or graph is continuous and proportional to the hardness. It is not an ordinal scale like the case of Mohs. and not represented by an integer – Photo credit: Eurico Zimbres, Wikimedia, CC BY-SA 2.5
Are you concerned that the Mohs method doesn’t have a basis? You don’t have it. Yes, the techniques are different but measure the same thing in essence, i.e., Mohs scratch and the various indention methods test the hardness of materials by determining the resistance to dislodge atoms from their position. The abrasive force or indention pressure measure tells you how easy or hard it is to break bonds or dislodge atoms.
Frequently asked questions?
What is the softest and hardest mineral on the Mohs hardness scale?
The softest mineral on the Mohs Hardness Scale is talc, represented by 1, while the hardest is diamond, represented by a 10. However, there are a few minerals below talk and above diamond in hardness. This scale does not consider solid or gaseous materials.
Are there minerals softer than talc?
Yes. Minerals like cesium, lithium, sodium, potassium, rubidium, and candle wax are softer than talc. Luckily, you are unlikely to require a hardness test to identify them.
Are there minerals harder than a diamond?
Yes. Whereas the Mohs Hardness Scale has diamond, number 10, as the hardest, there are rare, harder minerals. Examples of such minerals are Wurtzite Boron Nitride and Lonsdaleite. The latter is a hexagonal diamond.
What determines the actual hardness of minerals or rocks?
The true hardness of any material or mineral depends on the purity, bond strength, molecular structure, or crystallinity structure.
Summary
Mohs Hardness Scale or Test tells you more about minerals and other material’s hardness based on which scratches the other or scratchability. So, the characteristic or property it tests is hardness. As a rule of thumb, the harder materials resist scratching or abrasion. So, it will scratch the softer one, and consequently, they rank her.
On the other hand, the less hard or softer material gets scratched easily. So, it ranks lower on this scale.
