The Mohs hardness scale was devised by Friedrich Mohs
in 1812. It basically is a loose and fast way of determining the relative hardness
of minerals and other substances. In spite of not being very quantitative, it's still used extensively.
It is very important to consider what we mean with hardness in this context. The hardness of a mineral refers to the surface hardness only. In practice, this means the resistance to mechanical surface damage. It has no bearing on things like brittleness or tensile strength.
Mohs' idea of the scale came from his observation that a harder material will scratch a softer material. He calibrated this to a scale of ten minerals, each capable of scratching the minerals above them:
The scale is not linear, for instance diamond is several times harder than corundum.
As said, it is possible to determine the hardness of a substance by scratching it with materials on this list. If you want to do this, and don't carry a mineral cabinet with you, you could do the following things to help you determine the hardness:
- Materials with hardness 1 can be ground with just your fingers
- Materials with hardness 2 can be scratched with your fingernail
- Materials with hardness 3 can be scratched with a copper coin
- Materials with hardness 4 and 5 can be scratched with a knife or a glass shard
- Materials with hardness 6 can be scratched with a steel file
- Materials with hardness 7 and up give sparks when you beat them against steel
- Materials with hardness 8 to 10 are fairly rare
Typically, the hardness can be determined up to half a point in the scale.
As mentioned, the Mohs scale is not very quantitative. This is not as big a handicap for the determination of minerals as it might seem. The hardness of a material is determined by its surface properties, and those are often not as well-defined as its bulk properties, because of external influences. This means, that the hardness of minerals you find might deviate a bit from what you expect. By browsing through Mineralen en Gesteenten, it seems that deviations of half a degree are common.
Hardness is a very desirable quality in gems. It would be an enormous shame to see your precious polished stone all scratched up just because you brushed it against sand or steel. And indeed, most of the valuable gems are hard:
while less valuable gems tend to be softer:
If you need to know the hardness of a material exactly, for instance for engineering purposes, there are various standardized tests, such as the Knoop hardness, Vickers hardness , Rockwell hardness and Brinell hardness hardness test. These are not difficult to perform, although they require a dedicated apparatus.
- J.Kourimsky, Mineralen en Gesteenten, Michon 1994.
- W.D.Callister jr., Materials science and engineering:an introduction.