A penny is sometimes used as a hardness test standard.
The concept of hardness is difficult to define readily when discussing rocks and minerals. This is because minerals are complex chemical compounds and may exhibit different characteristics in response to different forces, such as scratching, bending or hammering. Geologists have decided to base hardness on resistance to scratching, which is a measure of the relative strength of atom-to-atom bonds within a mineral. By this definition, one mineral is considered harder than all others.
Concept of Hardness
In geology, the hardness of a substance is defined very specifically as how hard that material is to scratch. A mineral that is resistant to scratching is therefore considered “hard,” even though it may be weak in other ways — for example, it may shatter when hit or break easily when stressed. The advantage of using scratch resistance as a test is that it does give a good measure of how soft a material is and is also quick and easy to perform, especially in the field.
The test used for hardness is that developed by the German scientist Friedrich Mohs more than a hundred years ago and is hence known as the Mohs test. The basis of the test is simple: If one mineral can produce a visible scratch mark on another, then that first mineral is harder and has a higher place on the scale. The Mohs scale places minerals in their relative rankings in order of hardess, but it is not a linear scale.
Testing for Hardness
The Mohs test is quite basic. You simply attempt to scratch one mineral with another. If that mineral can be scratched, it has a rank on the Mohs scale below the mineral that you used to test it. If it is not scratched, it is higher on the Mohs scale than the test mineral. The scale also allows for the use of several common scratch test items that are convenient and readily accessible most of the time. The are: the fingernail (hardness value of 2.5), a penny (3.5), steel knife blade (4.5), glass (5.5) and quartz (7).
The Rank of Minerals
The minerals identified by Mohs as corresponding to hardness values of 1 through 10 on this scale are as follows.
By this ranking, then, the hardest known mineral is diamond. It is worth noting that since this scaling is not linear, diamond is actually much harder in comparison to the other minerals than it appears from its ranking of 10. In fact, diamond is about 40 times harder than talc, the softest mineral on the scale.