by Katarina
Mohs – Carl Friedrich Christian – was an old German fella, a geologist and mineralogist. He lived some time ago (1773 – 1839). You wonder why I mention Mohs? Well, he expanded on the knowledge from Theophrastus and Pliny the Elder who determined that Diamonds where pretty hard compared with other minerals such as Quartz.
Good old Carl used the knowledge from those who came before him and it became the basis of the hardness scale he developed, today known as Mohs’ Scale of (Mineral) Hardness.
When Mohs developed his hardness scale in 1812, very little information about mineral hardness was available. He simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10. It was a relative scale in which a mineral of unknown hardness could be tested against a group of ten index minerals to see where it positioned on the scale.
You most certainly have heard talk around the club house of how hard a piece of rock / mineral / gemstone is when people want to polish them.
This hardness is actually measured by the Mohs Scale of Hardness. The scale measures the scratch resistance of minerals, how difficult it is to scratch a specific mineral. This has an impact on how difficult it is to polish one of our pieces into a beautiful cabochon or faceted stone.
The highest resistance to scratches is found on diamonds and they have been assigned the hardness 10. The least resistance to scratches is talc – it basically scratches itself – and it therefore has the hardness of 1.
The higher a mineral is on the scale, the harder it is to polish. And the reverse works of course too, the lower the ranking, the easier it is. Which of course means you could polish a low ranking mineral away to nothing in no time.
A good indicator to how hard a mineral might be is sawing a piece. The more the saw has to work, the harder it usually is.
But all in all, the scale doesn’t mean much. After all, what does it mean for example, that an Amethyst has the hardness 7 if you have nothing to compare it with? The numbers on the scale are just that, numbers.
So I have searched the net and asked my friend Google if there are any examples for the hardness. And as Google knows everything, it has delivered. The combined findings are in the table.
One of the comments that I remember is that Stones with hardness of under 4 may not polish at all or will only take a light shine. Most of the stones hobbyists polish are found in the 6 to 7 range.
As you will see in the table, many minerals don’t have a specific hardness but rather sit on a sliding scale, depending on the composition of the minerals.
And if you ever find yourself in Vienna, why not look for the memorial plaque in honour of Carl Mohs.
Sources:
https://www.gemsociety.org/article/select-gems-ordered-mohs-hardness/ – here you will find a more complete list of minerals and there hardness according to Mohs
https://geology.com/minerals/mohs-hardness-scale.shtml – more in depth information. Including how to test minerals.
https://en.wikipedia.org/wiki/Mohs_scale_of_mineral_hardness
