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The Mohs' Scale of Hardness

Why is the Mohs’ Scale important to you as a Jewelry Wearer?

 By Gary Hocking


Hardness can be tested through scratching so Friederich Mohs devised a scale of hardness from 1- 10. This scale was actually a scale where the softer mineral on the scale could be scratched by the one above it.


 Most of us are aware that diamond is the hardest gemstone. Diamond is the hardest on the scale and is rated as having a hardness of 10.


Let me tell you from a jeweller’s perspective what that means.  I buy some very expensive files perhaps ten times more expensive that what you will buy at your local hardware shop.  When I accidentally file a diamond you might think that I have made a big mistake and that I will file a piece of the diamond and make it dull.  Well the reverse is the case as the diamond will blunt my expensive files!


The next down the hardness scale is Corundum which is what sapphire and ruby are.  These gemstones are really the same thing.  When a sapphire is red then we call it a ruby. These are nine on the scale.


The scale goes all the way down to one and this is talc which is what talcum powder is made from.


So let’s get to the point.  Why should you care about the Mohs’ scale of hardness?  Well, you want to know where your favorite gemstone is on the scale and this will give you an idea of its limitations.


My favorite gem is the opal.  This has a hardness of around 6.  That is not very hard at all.  It’s about the same as glass. That means you can scratch it with something such as a knife.


Opals are not facetted stones. Why not?  Firstly, because they are generally translucent or opaque stones and the light cannot enter them so faceting is pointless.  Secondly, because they are soft stones and the facets would be easily chipped and damaged.  The style of cutting is called a cabochon that we use for opals and other soft stones.  It is a dome which kind of looks a bit like a bowler hat and this is where the name comes from, an old French word for hat.


If you choose a stone such as an opal with a low hardness then you should put it in a setting where it will not get damaged.  My mother is eighty years old and she is gentle enough to wear an opal ring everyday of her life and she will never damage it.  But most people who want to do the dishes wearing their opal ring will soon damage it.  You shouldn’t be wearing an opal ring to the office everyday.  If you are not like my mother then you should be thinking of putting that expensive opal into a pendant or earrings.


If you are have been given  a pearl which was found in an oyster by your great grandfather and you really want to make it into a ring and wear it all the time then you should think about the fact that it comes between 2.5 and 4 on the Mohs’ scale.  That means that it is a soft stone and will get severely damaged if you are a chef but will just fine if you are a concert pianist.


What if you inherit an emerald?  Well, an emerald is hard enough for a ring but it won’t take the punishment that a diamond or a sapphire will so it might be better to wear it as a pendant if you are going to be very active.


What about coral?  Again it is good to know that you are dealing with a very soft item and it might be best as earrings.


So the Mohs’ scale will give you a quick indication of whether you should buy certain stones at all and what jewelry you should have it made in.

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