As an Amazon Associate, I earn from qualifying purchases with no additional costs for you.
The mineral’s hardness is one of the most helpful characteristics in mineral identification. The advantage of the hardness test is that it can be easily done at home or even in the field while rockhounding.
Testing a mineral’s hardness is one of the keys to mineral species identification. Mineral’s hardness test can be easily held at home or during a rockhounding field trip. There are necessary tools for express hardness test: fingernail, cooper coin (penny or 1,2,5 cents euro), a piece of glass, a knife blade.
Two necessary tools for the Mineral Hardness Test are already with you! They are your fingernail and coins in a pocket. Further, we will shed a light on what is the Mohs Scale and how to apply the knowledge from the scale to quick mineral identification using common stuff everyone has. No worries, nothing extraordinary like unicorn horn is needed. By the way, geologists like the hardness test the most!
If you are interested in checking out the best Mohs hardness test kits only you can find them by clicking here (Amazon link).
Why is Mineral Hardness Important?
Mineral hardness gives an understanding of how resistible is mineral to scratching. Each mineral has its definite hardness. So being able to do a hardness test sets up a fast way to mineral identification.
Mineral hardness is an important characteristic as it reflects the chemical composition and the structure of minerals. Knowing the mineral hardness narrows down a list of probable minerals during the sample identification. The hardness tests can easily differentiate two very similar-looking minerals.
How Do You Determine the Hardness of a Mineral?
The hardness test doesn’t require any complex equipment. It can be easily held even at home with very common tools everybody has. Stay tuned, and you will receive step-by-step instructions.
To determine the hardness of a mineral – do the Mohs Hardness Test. Scratch the mineral with a tool with known hardness. Observe if a tool leaves scratches or not. If it does – the mineral is softer. If it doesn’t – the mineral is harder. Compare the result to the Mohs Scale and find a possible mineral.
How to Do a Mohs Hardness Test?
The Mohs hardness test is just an official name of the hardness test. Introduced in 1822, the scale originated when Friedrich Mohs chose ten minerals and assigned numbers to them based on the relative ease or difficulty with which one can be scratched by another. You will see his studies resulted in the scale below, which is still used today.
The Mohs hardness test is done by scratching a mineral sample by a tool or other mineral with known hardness. If quartz (7 on the Mohs scale) cannot scratch your sample, it means your mineral is harder than 7. If your sample cannot scratch a piece of glass – the hardness of your mineral is lower than 5.
The Mohs Scale ranks minerals and gem’s hardness on a scale of 1 (least hard – talc) to 10 (hardest of all – diamond).
The Mohs hardness scale:
| Mineral | Hardness |
|---|---|
| Talc | 1 |
| Gypsum | 2 |
| Calcite | 3 |
| Fluorite | 4 |
| Apatite | 5 |
| Feldspar (sometimes indicated as Orthoclase) | 6 |
| Quartz | 7 |
| Topaz | 8 |
| Corundum (ruby and sapphire) | 9 |
| Diamond | 10 |
It’s vital to understand that the Mohs scale is not a linear scale. Instead, it ranks minerals on a relative scale based on their scratch hardness, so although corundum (ruby or sapphire) is a 9, a diamond at 10 is almost 5 times harder (according to Vickers Absolute Hardness Scale) and 3 times harder (according to Knoops Absolute Hardness Scale)!
According to the Mohs Scale, only a diamond can scratch a diamond. Corundum can scratch itself, topaz (8), quartz (7), and anything softer. Topaz can scratch itself, quartz (7), and anything lower on the scale.
Important notification!
The hardness of a stone indicates the stone’s resistance to scratching or how the surface of the mineral will respond to contact with a sharp point. This differs from a mineral’s toughness, which is defined by how well a mineral can survive an impact or resist breaking, chipping, or cracking.
Consequently, a diamond can be easily shattered when struck with a hammer since the diamond is brittle. However, the diamond cannot be scratched with the same hammer. Toughness and hardness are different physical characteristics.
TIP: A Mohs hardness test is one of the most useful tricks that should be in all rockhounds’ playbooks. Check out the best test kits in the article below:
3 Best Mohs Scale Test Kits: Test Hardness of Your Gemstones
How Do You Test the Hardness of a Mineral (at home)?
The hardness of minerals can be easily tested at home. You will just need some simple tools with known hardness and a pinch of patience.
To perform the hardness test, you will need your fingernail, cooper wire or coin, a piece of glass, a knife blade, and a steel file. All of them have known hardness comparable to the Mohs Scale. Scratch your mineral with the tools described above. Harder tool leaves a scratch on the softer mineral.
Here is a step-by-step instruction on how to test the hardness of a mineral at home:
- Prepare something resistible to protect the floor or a table surface where you are going to conduct a test. Your mineral can be hard, but tabletop can not.
- Put your unknown mineral sample and fix it firmly.
- Scratch your mineral with a fingernail. If you see a scratch on the mineral surface, it means that mineral hardness is below 2.5. Compare this hardness with the Mohs Scale. Your mineral is very soft, and most probably, it is talc or gypsum.
- If there is no scratch after a fingernail, try copper wire or a copper coin (penny or 1,2,5 cents euro). Cooper’s relative hardness is equivalent to 3 on the Mohs scale. It will definitely scratch minerals below 3. With pressure applied, it can scratch fluorite also.
Important note! It is reported that coins nowadays are made of zinc (penny) or are copper-coated (1,2,5 cents euro). So it’s better to use copper wire.
- A very crucial step is a test with a piece of glass because glass hardness is in the middle of the Mohs scale. It allows separating minerals whether they belong to the lower or the higher part of the Mohs Scale. In the case of a piece of glass, it is better to do the test the opposite way. Take the mineral and try to scratch a piece of glass. If a mineral can scratch it – the hardness of the unknown mineral is more than 5. Therefore it can be feldspar, quartz, topaz, corundum, and diamond. If a mineral cannot scratch a piece of glass – it is softer than 5.
- A common knife has a very similar hardness to glass. So it can be also used to differentiate whether a mineral is softer or harder than 5.
- A steel file is a bit harder (6 – 6.5) than a knife blade (5 – 5.5). It means a steel file can scratch feldspar but will not scratch quartz.
- Quartz itself is also a good tool for hardness tests as almost every mineral lover has quartz crystal in their collection. So quartz will scratch everything below 7. If quartz cannot scratch an unknown mineral, we have good news for you! Most probably, you have topaz, corundum (ruby and sapphire), or even a diamond.
- Diamond can scratch any mineral. Diamond is the hardest. But remember – it is not the toughest.
Important note! Please, be careful if you use jewelry with diamonds for the hardness test, as they can chip because of pressure applied.
Another important note! As you can see, different minerals and gemstones have different hardness. Therefore it is recommended to store minerals and pieces of jewelry separately to avoid scratching.
BTW: Do you want to know more about rock and mineral identification? The books listed below are the best ones you can find on the internet (Amazon links):
- Smithsonian Handbooks: Rocks & Minerals
- Gemstone & Crystal Properties (Quick Study Home)
- Ultimate Explorer Field Guide: Rocks and Minerals (National Geographic Kids)
Some tips for a successful hardness test:
- The scratch test is not a rubbing test. No need to rub glass onto minerals. This can confuse you, as it is hard to notice whether is it a scratch or chalky particles of the tool you used. It should be one directed scratch.
- Also, it is a good practice to wipe a scratch and to remove white chalky particles created. This will let you observe a scratch clearly.
- Additionally, sometimes it is very helpful to use a magnifying glass to observe a scratch. There are some cases when it’s hard to understand whether it is a scratch or a chalky sign.
- The scratch test can be done on minerals or, exclusively, on very coarse-grained rocks. As common rocks are composed of less than 1 cm minerals, it is challenging to test so small minerals. Additionally, minerals in rock have different hardness. Testing mineral hardness included in rocks requires additional skills.
- There is a special mineral – kyanite, that has hardness anisotropy. It means it has a different hardness in different directions: 4.5 to 5 if tested parallel to the length of a crystal; and a hardness of 6.5 to 7 if tested across the short dimension of a crystal. Therefore test the hardness in different mineral directions.
TIP: Licking rocks and minerals in geology is a common practice to define some kind of minerals and, of course, a bunch of fun. Find out how to lick your rocks like a pro in the article below:
Licking Rocks in Geology: Why & How (Answered by Geologist)
What Tools Are Used to Test a Mineral Hardness?
Tools for a mineral hardness test are very simple. Everybody has it. You just need to know their equivalent hardness to the Mohs Scale to perform the test. Further, you will find a clue.
Common tools to test a mineral hardness are a fingernail, cooper wire or coins (penny or 1,2,5 cents euro), a knife, a piece of glass, and a steel file. Additionally, a set of minerals with a known hardness can be used. For example, quartz crystal, which hardness is 7, is also very helpful.
Here is a table of various tools’ hardness equivalent to the Mohs scale:
| Tool | Mohs Hardness | Comment |
|---|---|---|
| Fingernail | 2 – 2.5 | A fingernail will scratch talc and gypsum but will not scratch calcite. |
| Cooper wire or coin | 3 | Copper cannot be scratched by talc, gypsum, and calcite but will be scratched by fluorite, apatite, feldspar, and quartz. |
| A piece of glass | 5 – 5.5 | A piece of glass scratches everything up to apatite. But will be scratched by feldspar, quartz, topaz, corundum (ruby and sapphire), and diamond. |
| Knife blade | 5 – 5.5 | A knife blade cannot scratch quartz and higher. |
| Steel file | 6 – 6.5 | A steel file can differentiate between feldspar and quartz. A steel file scratches feldspar but cannot scratch quartz. |
TIP: If you are interested in checking out the best Mohs hardness test kit I recommend you can find it by clicking here (Amazon link).
Important note!
Materials described in the table above, especially a piece of glass and a knife blade, can have different hardness depending on composition, impurities, alloy variations, and manufacturing process.
For a more precise test, it is better to use the Mohs Mineral Hardness Test Kit, which is composed of 9 minerals (excluding diamond) from the Mohs Scale; or Hardness Pick Set. Hardness Pick Set is a set of calibrated pencil-looking picks composed of alloys carefully selected to match the hardness of the Mohs index minerals. It allows making a hardness test more precise.
Conclusion
Testing mineral hardness at home can be easily performed according to the Mohs hardness test. The Mohs Scale is ten minerals and assigned numbers to them based on the relative ease or difficulty with which one can be scratched by another where 1 (least hard – talc) to 10 (hardest of all – diamond).
The Hardness test is a comparison between what material can scratch another or not. Only a diamond can scratch a diamond. Corundum can scratch itself, topaz (8), quartz (7), and anything softer. Topaz can scratch itself, quartz (7) and anything lower on the scale, and so on.
Equivalent tools can come in handy if you don’t have minerals from the Mohs Scale. If a tool leaves a scratch on the unknown mineral, the mineral is softer than the tool.
Some common tools:
- fingernail (2 – 2.5),
- cooper wire or coins (penny or 1,2,5 cents euro) (3),
- knife blade (5 – 5.5),
- piece of glass (5 – 5.5), and
- steel file (6 -6.5)
are used for a Hardness test as they are equivalent to some values in the Mohs Scale.
TIP: If you are interested in checking out the best Mohs hardness test kit I recommend you can find it by clicking here (Amazon link).
TIP: Can you imagine 300 kg of rock moving in a desert and leaving tracks behind without any help? Sounds ridiculous. But it has a rational explanation. Find it out in the article below:
Sailing Stones Explained: Why & How Do They Move? (7 Facts)