Mohs Scale of Hardness

If you love diamonds, then you have probably heard it said that diamonds are the hardest mineral in nature.  This rock of ages got this reputation not because it’s “hard” to find, but because of a systematic means of testing it against other gemstones.

 

The Mohs Scale of Mineral Hardness gives a rating to various minerals by testing them for scratch resistance.  One mineral is scratched with another to determine which is harder:  The mineral with the least scratching after the test is the harder one.   A German geologist named Friedrich Mohs invented the scale in 1812.  However, this method (without the Mohs branding) has been employed all the way back to the days of ancient Greece.  The first mention of it was in the book “On Stones” in 300 B.C. by Therophrastus, a natural historian and successor to Aristotle as Athens’ lead philosopher.

 

The Mohs Scale has a rank order of 1 through 10, with talc, the softest mineral, at 1, and diamonds at 10.  This doesn’t mean that diamonds are 10 times harder than talc, however.  The true comparison is dictated by what’s called the absolute hardness of the mineral.  Talc has an absolute hardness of 1, while the next mineral on the Mohs Scale, gypsum, has an absolute hardness of 3 – meaning it’s three times as hard.  At the other end of the spectrum, diamonds have an absolute hardness of 1600.

 

Other minerals not on the scale fall anywhere between 1 through 10, including decimal places.  So if a mineral is scratched by (and thus softer than) topaz (8) but not quartz (7), it might have a ranking of 7.5, depending on the hardness of other similar minerals.  Since the original development of the scale, many more substances have been tested and given numbers that fall between the levels on the Mohs Scale.  These can be references for testing yet other minerals to see exactly where they fall.

 

Diamonds happen to be the hardest mineral and one of the most valuable, but the hardness of a gemstone isn’t a “hard and fast” rule for determining its value.   What it will tell you is how carefully you should treat the gemstone.  To get technical, diamonds are actually no longer the hardest mineral on Earth – that title belongs to two rare substances – mineral lonsaleite and quartzite boron nitride, both of which have a similar structure to diamonds, but are stronger and formed by chaotic geologic events.  Unless a meteorite strikes the ground or a volcano erupts – and someone happens to arrive with a pick axe – you’ll be on the waiting list for a long time.

 

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