Hardness is a measure of the ability of a material to resist wear and tear - ie, abrasion. The hardness of a mineral largely determines its durability and is related also to the strength and toughness of solid substances. It is important not to confuse hardness of a material with strength. Many hard materials (such as diamonds) are brittle and will break easily if they are struck. However, in common usage the term 'hardness' is often extended to include strength and toughness.

Strength of Materials

A full discussion of the strength of materials is beyond the scope of this Entry. Suffice it to say that there are different types of strength:

Hence, in simple terms, the strength of a material is a measure of its resistance to various forms of deformation.

Hardness

The relative hardness of minerals is determined according to Mohs Scale, named after the German mineralogist, Friedrich Mohs, who devised it in 1812. In the original Mohs Scale, ten minerals were arranged in order of increasing hardness and were assigned the numbers one to ten. These ten minerals are shown in the first column of the table below:

A substance with a higher Mohs number is capable of scratching a substance with a lower number.

The order of these minerals can be remembered using the mnemonic: The Girls Can Flirt And Other Queer Things Can Do .

Mohs selected these ten minerals because they were common or readily available. The scale is not a linear one, and is somewhat arbitrary. For example, fluorite at four is not twice as hard as gypsum at two; nor is the difference between calcite and fluorite similar to the difference between corundum and diamond.

Hardness is used in a rough way to inform mineral identification in the field. Real minerals out in the field can look remarkably alike. This may be due to weathering, variations in their chemical structure from the ideal, or clathrate inclusions that simply change the colour of the mineral. Sometimes faulting and metamorphism can induce facets and planes in a mineral that aren't natural to it, so that the mineral looks like another.

Common tests you can perform in the field if you don't have a material on the list, involve the convenient standards of hardness shown in the third column of the table above. Using this system, minerals may simply be classified as: soft (can be scratched with a fingernail), medium (can be scratched with a knife or glass but not by a fingernail) or hard (cannot be scratched by a knife). This simple classification is appropriate to use up to lower secondary school level.

Diamond is the hardest natural substance known and therefore scratches all common materials. Because of this it is widely used in industry for drilling, cutting, grinding and polishing. Diamond is an allotrope of carbon and it is interesting to note that another allotrope of carbon, graphite, is one of the world's softest minerals. This is why graphite is used in pencil 'leads'.

The Mohs scale is still used today although it has been extended, putting diamond at 15, to accommodate newly-developed materials of extreme hardness which lie between 10 and 15.

The Extended Mohs Scale

Industrial Measurement of Hardness

In metallurgy and engineering there are many more accurate and precise ways of measuring hardness. One way involves measuring the size of the dent made in a material under a particular pressure as applied by a pyramid-shaped diamond. For all but the hardest materials, a steel ball may be used instead of the diamond. The smaller the dent is, the harder the material. Hard metals indent less than soft metals. This test is known as the Brinell test, and is named after the Swedish engineer Johann Brinell. The instrument used to conduct this test is called a sclerometer, and this enables us to create an 'absolute hardness scale'.

An absolute hardness scale looks a little different from the relative scale. It turns out that adjacent minerals are relatively close in hardness; but as hardness increases, the difference in hardness increases greatly, as is seen in the scale below:

An absolute hardness scale

This scale shows that diamond is four times harder than the penultimate mineral, corundum, which itself is twice as hard as topaz.

Other Hardness Tests

Besides the Brinell Test, which employs a hardened steel or carbide ball indenter, there are two other hardness tests in common use: the Rockwell Test (involving a diamond cone-indenter or a hardened steel ball) and the Vickers Test (diamond indenter in the form of a right pyramid). Discussion of these tests are beyond the scope of this Entry, but may be found at this link.

Materials of Extreme Hardness

There is much interest in the development of super-hard materials - substances which are harder than diamond - for which there are many applications. Super-hard materials could be used to cut steel, which diamond cannot do because it burns when it gets hot. Although it is possible to coat metals with a thin layer of diamond, it would be somewhat easier with a synthetic material such as beta-carbon nitride. Mechanical components such as gears and bearings coated with beta-carbon nitride would last much longer than normal parts, and could be used in devices where liquid lubricants are unsuitable. A thin layer of the beta-carbon nitride could also be used to protect the surface of computer discs.

Another celebrated material is Borazon, a synthetic compound of boron and nitrogen, which was discovered by Dr R Wentorf in 1956. Depending on the source of information, Borazon is either harder than diamond or of equal hardness to diamond because each will scratch the other.