As jewellers, we often choose a gemstone based on the 4 C’s and of course, if it will look good in our design. But there are many more factors that need to be considered when purchasing a gem to be worn in jewellery. 

A major consideration should be the durability of the gemstone and if it is able to physically withstand the intended use of the finished jewellery.  Another factor would be whether you have the skills necessary to set it without breaking it in half.

Durability is composed of three entirely separate components; hardness, toughness and stability.  Each needs to be carefully considered when selecting a gemstone for a specific piece of jewellery.

Hardness is a materials ability to resist scratching and abrasions and in gemmology it is measured in terms of the Mohs relative scale of hardness.  The Mohs scale is a non-linear scale of common minerals numbered 1 to 10, with diamond being the hardest at number 10. Despite being only one place above corundum on the scale, diamond is many, many times harder. Diamond is the hardest natural material in the world.

1.    Talc
2.    Gypsum
3.    Calcite
4.    Flourite
5.    Apatite
6.    Orthoclase Feldspar
7.    Quartz
8.    Topaz
9.    Corundum
10.   Diamond

When selecting a gem for a particular project such as a ring that will be worn every day, careful consideration needs to be given to the hardness of the stone, as anything as soft as quartz or below will be scratched by dust particles of which quartz is a common component.   Also important to note is if a design contains many stones (especially of varying harness), they should not be allowed to rub against each other during storage or wear – for instance one must be very careful when combining pearls with harder stones in a design, if the harder stones are allowed to come into contact with the pearls during wear or storage, the pearls will soon be damaged – and scratched up pearls are about as pretty as an upset customer!

 











Toughness is a materials ability to resist breakage, and is totally different from hardness.  A piece of leather for example, is soft and easily scratched, however because it is tough, it is very difficult to break. A gemstone’s toughness is generally rated as poor, fair, good, very good, excellent – or other similar words depending on which gemmological authority you subscribe to.  Many tough gems are polycrystalline i.e. made up of many microscopic interlocking crystals. The toughest gems are the two stones commonly referred to as jade – jadeite and nephrite, as well as chalcedony (the polycrystalline variety of quartz).  Diamond has very good toughness, but it is a single crystal material and has a property called “cleavage” which means it is weaker in some atomic directions than others because the bonds between the atoms are not as strong in certain directions.  Topaz is a gemstone with only fair toughness as it’s very susceptible to breaking due to cleavage.  Other than cleavage, brittleness is also a big contributor to a gemstones toughness – gemstones such as zircon are very brittle and often get lots of tiny chips along their facet junctions if worn regularly.  Finally check the clarity of a stone, any major breaks already in the stone, may only need a slight tap in the future to extend them and break the stone in two.  Again serious consideration needs to be given to setting stones with low toughness in rings, if a stone is to be set in a ring, the style of setting needs to protect the stone from accidental knocks.  From the jeweller’s perspective, setting stones with low toughness requires a lot of care and attention as one false move could break the stone.

Stability is a gemstone’s ability to stand up to chemicals, heat and light.  Gems such as opal are very sensitive to heat and lack of moisture. In a warm and dry environment they can develop a tiny network of cracks called crazing, which cannot be repaired.  Other gems such as amethyst can be sensitive to direct sunlight which can cause them to fade over time.  A gemstone’s stability is generally rated similar to that of toughness – again with slight deviations depending on which gemmological authority you are listening to.

It’s important for all jewellers to know the basic details about the gems they are working with and how they will withstand their intended use - for both their own sanity when trying to set evil topaz and also to inform the customer on how to care for their new jewellery.  For further reading, I would recommend a book called “Working with Gemstones…..  A Bench Jewelers Guide” by Arthur Anton Skuratowicz and Julie Nash.

Lucy Ryalls

44 Hockley Street

Hockley

Birmingham

B18 6BH

Tel: +44 (0)121 507 0994

Company number 07258309