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Mohs Scale of Mineral Hardness

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Mohs Scale Measures Hardness not Toughness

 

Mohs scale of mineral hardness was created in 1812 by Friedrich Mohs, a mineralogist from Vienna. Its purpose was to identify 'scratch hardness' which is the resistance of one mineral when being scratched by another.

Mohs scale was made up of ten common minerals all of which had a different level of hardness.  The softest was the clay mineral talc and the hardest was diamond. Still referred to today as Mohs scale of mineral hardness, it's widely used and demonstrates that talc which grades 1 on the scale can be scratched by gypsum which is grade 2. Gypsum in turn can be scratched by calcite because calcite is harder than gypsum hence calcite is graded as 3. The harder the mineral the higher the grade and with diamond being the hardest of all minerals it's grade is 10.

Every mineral on the Mohs scale of mineral hardness can scratch another that has a lower grade and can in turn be scratched by one that's higher. Minerals of the same hardness can scratch each other.  Half grades are also used and all known minerals have a place on the scale.

The concept is pretty logical, for one material to be able to scratch another it must be harder.  Take a nail file for example, if the abrasive coating the file were to be softer than your nail it would have no effect.  Nail files are often coated with corundum which after diamond is the hardest mineral known to man.  Ruby and sapphire are gemstone varieties of the mineral corundum.  Although there's only one grade on Mohs scale between corundum and diamond, diamond is four times harder than corundum.

Whilst scratch resistance identifies a mineral's hardness it's not the same as toughness.  The correct geological term for toughness is tenacity and describes a mineral's resistance to cracking, chipping or pressure.  Diamond which is the hardest of all minerals can't even be scratched with a knife but drop it onto a hard surface and it will very likely break.  Pearl however which is at the other end of Mohs scale and grades just 2.5 can be very tough so will not chip easily and is unlikely to break if dropped.


 

 

Mohs Scale of Mineral Hardness


Hardness

Mineral

Examples

1

Talc

Talcum powder

2

Gypsum

Selenite

3

Calcite

Calcite

4

Fluorite

Fluorite

5

Apatite

Another example is Turquoise

6

Orthoclase Feldspar

Labradorite Moonstone Sunstone Unakite  Amazonite

7

Quartz

Amethyst Citrine Rose Quartz

8

Topaz

Topaz

9

Corundum

Sapphire Ruby

10

Diamond

The hardest of all natural minerals

 

 

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