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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.

The purpose of the scale was to identify scratch hardness.  This is the resistance of one mineral when being scratched by another.

Mohs scale was made up of ten common minerals all with a different level of hardness.  The softest was talc the hardest was diamond.

The scale is still widely used today and is still referred to as Mohs scale of mineral hardness.

The scale shows the mineral talc which grades 1 can be scratched by gypsum which grades 2. Gypsum in turn can be scratched by calcite which grades 3.

The harder the mineral the higher its grade.  With diamond being the hardest substance known to man it grades 10.

Every mineral on the scale can scratch another with a lower grade.  It can also be scratched by one with a higher grade.  Minerals with the same hardness can scratch each other.  Half grades are also used. 

All known minerals and most rocks now have a place on the scale. 

The concept is really quite logical.  For one material to be able to scratch another it must be harder.  Using a nail file for example, if the abrasive on 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.  Although there's only one grade on Mohs scale between corundum and diamond, diamond is four times as hard.

Whilst scratch resistance identifies a mineral's hardness it's not the same as toughness.  The correct geological term for toughness is tenacity.  This describes a mineral's resistance to cracking, chipping or pressure.

Diamond can't be scratched by any other mineral but drop it onto a hard surface and it's highly likely to break.  Do the same with a pearl which grades just 2.5 on Mohs scale and it's unlikely to break or even chip.

Mohs Scale of Mineral Hardness

Hardness

Index
Mineral

Examples

1

Talc

Talcum powder

2

Gypsum

Selenite

3

Calcite

Calcite

4

Fluorite

Fluorite

5

Apatite

Turquoise

6

Orthoclase
Feldspar

Labradorite
Moonstone
Sunstone
Amazonite

7

Quartz

Amethyst
Citrine
Rose Quartz

8

Topaz

Topaz

9

Corundum

Sapphire
Ruby

10

Diamond

Hardest substance
known to man

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