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The Mohs scale is a system of testing the hardness of a mineral, designed by Friedrich Mohs in 1812. Mohs was a mineralogist from Germany who wanted a simple way of testing the “scratching” ability of each mineral. What the mineral could scratch, or what could scratch the mineral, determines its position on the scale.
Mohs designed the scale to work with relatively ordinary devices. For example, one tested the hardness of a mineral with things like a fingernail, a penny, glass, or a knife. Even having access to only a few of these things in the field could help determine where the position of the mineral in relation to others.
On the traditional Mohs scale, the hardest mineral is the diamond. It cannot be scratched by another mineral and can scratch every other mineral. Its hardness is measured as the number 10. Some minerals fell in between the numbers of 1-10, however, and scientists proposed a revised scale that rates hardness up to 15. This allows some minerals that fell into grey areas to be reclassified.
The scale does not measure absolute hardness, and each number does not quite correspond to double the strength of the next number up. For example, talc has a hardness of 1 on both the Mohs and the absolute hardness scale. The next mineral up, gypsum, is twice as hard and absolutely hard.
Large differences occur as the scale progresses. For example a diamond, rated 10 on the older Mohs scale, has an absolute hardness of 1500. Thus it is 1500 times as hard as talc. Quartz, listed as number 7, has a 100 rating of absolute hardness. This shows that the numbers on the scale don’t translate to twice as hard, or three times as hard.
The Mohs scale instead offers a hardness measurement that is relative to other objects. It’s still a fairly easy system to learn, and one can soon practice mineral measurements with little difficulty. However, gaining access to certain minerals such as diamonds may be difficult for the junior mineralogist.
The hardness of minerals can also vary depending on the direction that the mineral is tested. Hardness Testing Pencils are available online.