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The Mohs scale and the hardness of gemstones

Hardness is one of the main properties of minerals and is especially relevant to those working with gemstones, for example, in jewellery and gem cutting.


But what is the hardness of gemstones?


Well, according to the International Gem Society, hardness refers to the resistance or opposition of a mineral to being scratched.


In practical terms, this means that harder gemstones do not scratch “as easily” compared to softer or less hard gemstones.


For example, sapphires are harder than sphalerite and therefore scratch less easily.


Sapphire is one of the hardest gemstones.
Sapphire is one of the hardest gemstones. Photo: Carolina Vivas-Serna

Knowing the hardness of minerals is important so that goldsmiths can select the most appropriate gemstone for a particular jewel.


Additionally, it allows gem cutters to choose the appropriate grinding blades, polishing discs and abrasives for each cutting process.


But how is the hardness of a gemstone measured?


To understand it better, in this article we are going to explain:



Let's get started!


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What is the Mohs scale?


The Mohs scale is a ratio of ten different minerals, which was developed by the German geologist and mineralogist Friedrich Mohs in 1812.


The scale is represented by a table that goes from 1 to 10, in which each mineral occupies a position depending on the value of its hardness.

However, it is important to note that the Mohs scale provides a relative hardness value to make its use more practical.


For example, the absolute hardness of corundum is 400 and the hardness of diamond is 1500. However, Mohs gives them a relative hardness of 9 and 10 respectively, to keep it simple.


As you will see below, it all boils down to the fact that a hard mineral can scratch a soft one, but a soft mineral cannot scratch a hard one.



How is Mohs scale calculated?


To understand how the Mohs scale works, it is necessary to understand what it means for one mineral to scratch another mineral.


But to be practical, let's use an example with totally different materials: an orange brick and a steel knife.


As the steel knife is hard, it is capable of scratching the surface of the brick. However, the brick cannot scratch the surface of the steel sheet.


The same thing happens with minerals, depending on the value of their hardness:

  1. Talc is scratched by all minerals (and even by a fingernail!).

  2. Gypsum is also scratched by a fingernail, but with more difficulty.

  3. Calcite is scratched by a copper coin.

  4. Fluorite is scratched by a steel knife.

  5. Apatite is scratched by a knife, but with more difficulty.

  6. Orthoclase is scratched by steel sandpaper.

  7. Quartz scratches glass (and from here the hardness increases!)

  8. Topaz is scratched by tungsten carbide tools.

  9. Corundum is scratched by silicon carbide tools.

  10. Diamond can only be scratched by another diamond.


In summary, we say that talc is the softest mineral on the Mohs scale since its hardness is 1 and it can be easily scratched with a fingernail.


Its opposite is the diamond, which has a relative hardness of 10 and can only be scratched by another diamond as it is the hardest mineral of all.


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Mohs scale table


In the following table you can see the main characteristics of minerals on the Mohs scale:


The Mohs scale and the hardness of minerals
The Mohs scale and the hardness of minerals

It is important to note that the Mohs scale is not linear or proportional, since hardness increases significantly when moving from one mineral to another.


That is, if you look at the absolute hardness you can notice that diamond is not only 10 times harder than talc, but 1500 times harder.



What is the Mohs scale used for?


The Mohs scale has educational and industrial applications, with these being some of its most common uses:

  • It is a reference used by students and fans of minerals, both in the field and in the laboratory.

  • It helps jewellers choose gems that are resistant to being scratched, depending on the jewel and the type of setting.

  • It is used in the construction industry since some minerals such as quartz are components of granite and other materials.

  • It is necessary for gem cutters to select suitable discs and abrasives for roughing and polishing each gemstone.


Cutting a malachite is not the same as cutting a corundum. Malachite is softer. A corundum like sapphire is harder, and a different, more expensive abrasive is required.

Process of cutting a malachite, a soft gemstone.
Process of cutting a malachite, a soft gemstone. Photo: Carolina Vivas-Serna

As you can see, the Mohs scale is still valid and its applications are still quite broad.


That said, it is time for you to know which the softest gemstones are.



Soft gemstones


Before starting, you have to know that there is no definitive classification that tells us exactly which gemstones are the softest.


But for practical purposes, we are going to consider that a soft gemstone is one that has a relative hardness equal to or less than 6 on the Mohs scale.


The explanation is simple:


According to the scale, minerals with a Mohs hardness equal to or less than 6 are unable to scratch glass (hardness of 6.5) or quartz (hardness of 7).


Furthermore, having a hardness of 6 or less, all of them can be scratched by knives, sandpaper or steel nails.


It is worth highlighting as exceptions to opal and moldavite in their hardest forms of 6.5 and 7 respectively.


What gems are considered soft?


As you will see below, the list of the softest gemstones includes minerals with hardnesses from 2 to 6 on the Mohs hardness scale.


We also include stones that may have higher hardness such as hematite, opal, rhodonite and moldavite because the range of their relative hardness starts around 5.5.

  • 2 – 2.5: seraphinite

  • 2 – 4: chrysocolla

  • 2.5 – 5: serpentine

  • 3: calcite, verdite

  • 3 – 3.5: howlite

  • 3.5 – 4: azurite, sphalerite, malachite, rhodochrosite

  • 4: fluorite

  • 4 – 4.5: smithsonite

  • 4.5 – 5: larimar

  • 5: apatite, hemimorphite

  • 5 – 5.5: sphene (titanite), lapis lazuli, obsidian

  • 5 – 6: charoite, turquoise

  • 5.5 – 6: cat's eye actinolite, scapolite, hackmanite, sodalite

  • 5.5 – 6.5: hematite, opal, rhodonite

  • 5.5 - 7: moldavite


Many of these soft stones are widely used in jewelry. However, they require greater care and delicacy compared to hard stones.


Honduran opal is a soft stone and can be used in jewelry.
Honduran opal is a soft stone and can be used in jewelry. Photo: Carolina Vivas-Serna

Now, let's talk about hard stones.


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Hard gemstones


Based on the relative hardness range, we decided to separate the following gemstones into two groups: the hard stones and the hardest stones.


Which gemstones are hard?


Hard gemstones include amazonite, jadeite, kunzite, peridot, garnet and quartz. Its relative hardness is medium – high, with values ranging from 6 to 7.5 on the Mohs scale.


These are some examples:

  • 6- 6.5: amazonite, benitoite, labradorite, moonstone, orthoclase

  • 6 – 7.0: cassiterite

  • 6.5: idocrase (vesuvianite)

  • 6.5 – 7: agate, axinite, chalcedony, chrysoprase, hiddenite, jadeite, kornerupine, kunzite, onyx, peridot, tanzanite

  • 6.5 – 7.5: zircon, garnet

  • 7: amethyst, ametrine, aventurine, citrine, quartz, jasper, tiger's eye


Now that you know more about hard stones, find out which are the hardest gemstones according to the Mohs scale.



What are the hardest gemstones?


Among the hardest gemstones we find tourmalines, beryls, spinel, topaz, corundums, moissanite and diamond. In this group, the minimum relative hardness is 7 and the maximum is 10.


Below we share a more complete list of the hardest gemstones in order:

  • 7 – 7.5: danburite, grandidierite, iolite, rubellite, tourmaline

  • 7 – 8.5: dumortierite

  • 7.5: andalusite, hambergite

  • 7.5 - 8: aquamarine, bixbite, emerald, heliodor, goshenite, morganite

  • 8: spinel, topaz

  • 8.5: alexandrite, chrysoberyl

  • 9: ruby, sapphire

  • 9.25: moissanite

  • 10: diamond


Iolite, with hardness from 7 to 7.5, is a very hard blue gemstone.
Iolite, with hardness from 7 to 7.5, is a very hard blue gemstone. Photo: Carolina Vivas-Serna

Suggested way to cite this article: Vivas-Serna, C. (date). The Mohs scale and the hardness of gemstones. Oscar Bautista. https://en.oscar-bautista.com/post/mohs-scale-hardest-softest-gemstones


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FAQ

1. What is Mohs scale?

2. How is Mohs scale calculated?

3. What is 10 on Mohs scale of hardness?

4. What gems are considered soft?

5. What is the softest gemstone?

6. What is the hardest gemstone?

7. What are the top 3 hardest gemstones?

8. What is the second hardest gem in the world?

9. What are the hardest gemstones in order?

10. Is there a gemstone harder than diamond?


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