Gemstones, also known as precious or semi-precious stones, are often described based on their rating on the Mohs hardness scale. The rating is one factor that determines what the stone can be used for—an everyday jewelry piece or a collector’s item for viewing.
Understanding how the Mohs scale of hardness works will help you take care of the gemstone, whether you’re wearing or storing it away.
So, what is the Mohs hardness scale, and how does it work? We answer these questions and more in this article.
What is Mohs’ hardness scale?
Mohs hardness scale is used to measure the hardness of a mineral. It is based on the mineral’s resistance to scratching or abrasion relative to 10 reference minerals.
The scale is used to identify minerals, especially in the field. While it may not always accurately identify them, it narrows down the options considerably.
💡 Take Note: The Mohs hardness scale is not linear, meaning that the increase in hardness is not in equal progression. The difference in hardness between a stone with a rating of 8 and 9 is less than that between a stone with a rating of 9 and a rating of 10.
Who created the Mohs’ scale of hardness?
Friedrich Mohs, a German geologist and mineralogist, developed the Mohs hardness scale in 1812.
Interestingly, Mohs was not the first to observe the difference in gemstone resistance to scratches. Ancient Greek philosopher Theophrastus mentioned it in his treatise, On Stones, in 300 BC. Roman author, Pliny the Elder also wrote about it in his book, Natural History, AD 77.
How to Use the Mohs’s Hardness Scale
The Mohs hardness scale ranges from 1-10, where 1 is the softest mineral which means it scratches easily, and 10 is the hardest, which means it can only scratch itself. The test is based on the fact that a harder material will scratch a softer one.
To use the scale, you should know that each mineral can only scratch those below it on the scale.
💡 Pro Tip: Don’t perform the scratch test on your gemstone’s surface in a noticeable area. Choose an area that you can easily hide.
To test for the hardness of a mineral, start with the softest mineral on the scale, Talc. Hold the gemstone to be tested and attempt to scratch it with each mineral of known hardness.
If the mineral is harder, it will leave an etched line on the surface. For example, if the mineral of known hardness is topaz with a rating of 8, it means that the unidentified gemstone has a hardness less than 8.
💡 Tip: Don’t mistake the powder streak of the mineral on the gemstone you’re testing for a scratch. Make sure to dust off particles before checking for a scratch.
Mohs Hardness Scale
The Mohs hardness scale runs from 1-10. Since most people do not carry the 10 reference minerals around, there are some everyday objects of known hardness you can use to estimate the hardness of your mineral.
| Mineral Name | Scale Number | Common Object |
| Diamond | 10 | |
| Corundum | 9 | |
| Topaz | 8 | |
| Quartz | 7 | Streak Plate (7) |
| Orthoclase | 6 | Steel Needle (6.5) |
| Apatite | 5 | Window Glass/Pocket Knife/Glass Plate (5.5) |
| Fluorite | 4 | |
| Calcite | 3 | Copper Penny (3/3.5) |
| Gypsum | 2 | Finger Nail (2.5) |
| Talc | 1 |
FAQs on the Mohs Hardness Scale
Got more questions about the Mohs hardness scale? Here are some of the most commonly asked ones.
What hardness is glass?
Glass has a hardness of about 5.5 – 6 on the Mohs scale of hardness. It is one of the everyday objects you can use to estimate the hardness of a mineral.
What is the hardest mineral on Mohs hardness scale?
The hardest mineral on the Mohs hardness scale is diamond, with a rating of 10. It is one of the reference minerals on the scale. It can scratch all other minerals on the scale and is about four times harder than the immediate mineral below it, corundum.
What is the second hardest mineral according to Mohs scale of hardness?
The second hardest mineral according to the Mohs scale is corundum. It is one of the reference minerals on the scale and has a rating of 9. Ruby and sapphire are two main gem varieties of corundum. They are harder than emerald, which has a rating of 7.5-8 on Mohs scale of hardness.
What is the weakest gemstone?
Talc is the weakest gemstone with a hardness of 1 on the Mohs hardness scale. It cannot scratch any other mineral on the scale but can be scratched by them all. You can also scratch it with your fingernail.
What’s your gemstone’s scratch resistance?
Mohs hardness scale is an important scale used to identify minerals. It is used alongside other characteristics, including color, refractive index, inclusion, luster, and dispersion.
Although other hardness test methods have been developed and used over the years, Mohs scale has remained popular for over 200 years because it’s affordable and easy to use.
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