The Mohs Scale

Joanne Tinley Author

What is the Mohs Scale, and most importantly, why is it useful to jewellers?

The Mohs' Scale of Mineral Hardness, to give it it's full name, was devised by a clever mineralogist named Friedrich Mohs. It is a relative scale of “scratchability.” Basically the higher the number on the scale the harder the stone. A stone rated at an 8 can be scratched or damaged by a stone rated at a 9, but not one rated at a 7. Mohs originally had just ten minerals on his scale - talc at the bottom, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum (i.e. sapphires and rubies - remember that ruby is a red sapphire!), and for last and hardest, diamond.

The scale was devised in 1812, and we know now that it is not completely accurate, but it is a very useful indicator of how much abuse a gem can take - or how much care is needed to keep it in good condition! As a general rule gemstones of a hardness of 8.0 or higher are considered to be durable. Intermediate stones are over 6.0 and up to 8.0, softer and more delicate stones are 6.0 and under. Gems that are 6.0 and under should be handled and cleaned with more care, and not exposed to chemicals or solvents.

There are always exceptions to every rule, however, and the main one in this case is emerald. Although it’s on the scale at 7.5 – 8.0 there are almost no perfect emeralds found. They have numerous inclusions (flaws) that lowers their resistance to breaking and so they should be treated with care. In fact, it is important to remember that no matter how hard a stone may be on the chart, the quality of the stone also determines its strength. A flawed diamond will be more vulnerable and more brittle than one that is absolutely perfect.

So that’s what the Mohs scale is….but why is it important to jewellers? I’ll give you three great reasons!

Material Selection: The Mohs scale helps you choose the right stones for your jewellery. For everyday wear, stones with a Mohs rating of 7 or higher are recommended, as they're more resistant to scratches and damage. One of the reasons that diamonds, rubies and sapphires are so popular in engagement rings is their durablity.

Design and Crafting: Knowing the hardness of the gemstones influences how they are cut, set, and polished. Softer stones require more delicate handling and are often set in protective settings. Pearls are often set inside a cup, for example, to help shield them from accidental knocks.

Maintenance and Care: The scale reminds us how to care for different types of jewellery. Softer stones, especially pearls and amber, need gentler cleaning methods to prevent damage. A question that I frequently get asked is whether jewellery with stones set in it can be polished in a tumbler. Personally I would always prefer to tumble polish jewellery before setting any stones as any stone can have hidden flaws, regardless of it's position on the Mohs scale, and I would not tumble a stone under a score of 8.

Over the years more gemstones have been added to the Mohs scale, as you can see on the infographic here, making it more useful to jewellers. Where does your favourite gemstone sit on the scale?

Want to find out more about using gemstones in your jewellery?

Here are a useful blog post and links to two tube setting classes.

Start with a simple hammered ring and then learn how to set sparkling gems in smooth modern settings	How to repair your burnisher - and why you need to!	A new tube setting project - including tips for setting oversized stones
The Tube Set Ring class  teaches you everything you need to know to create your own beautiful settings from simple lengths of tube.	A burnisher is a must-have for stone setting - and for many other jobs too! However, burnishers must be looked after carefully if they are going to give you the best results. To find out why and how to look after them read on!	A second tube set ring class - with a twist!  This class uses the stone  setting techniques taught in  the Tube Set Ring video to create a stone set between  two sides of a ring shank that twist past each other.

Categories: did you know, jewellery facts, stone setting