When looking for information about gemstones, you will commonly see their hardness mentioned such as 10 for Diamond or 8 for Topaz but this is only one of three factors that make up gemstones durability, a breakdown of the three are:
Hardness is how hard the surface of the gemstone is and the most commonly used reference point for hardness is the Mohs scale, that runs from 1 (Talc) through to 10 (Diamond). A simple definition for hardness is:
The harder the stone, the better its ability to resist scratching and abrasion.
|Corundum (Ruby & Sapphire)||9|
Toughness and hardness are often thought of as the same thing when it comes to gemstones but they are different as toughness is the internal strength of the stone and how well it can resist fracturing (non-directional breakage) and cleavage (directional breakage).
Unlike hardness, there is no recognised scale for this but it generally ranges from poor (Tanzanite, Emerald) to excellent (Jade).
Stability is the ability of the gemstone to resist physical (heat, light) and/or chemical (exposure to acids) changes. As with toughness, there is no recognised scale for this but it range from poor (Opal, Pearl) to excellent (Diamond).
Learning more about gemstone durability will make working with gemstones and in the table below are some of the most popular gemstones used in jewellery and their hardness, toughness and stability, listed in alphabetical order.
|Cubic Zirconia||8 1/4||Good||Good|
|Garnet (Demantoid)||6 1/2||Poor||Good|
|Garnet (Pyrope & Almandine)||7 1/4 – 7 1/2||Good||Good|
|Garnet (Tsavorite)||7 1/4||Good||Good|
|Jade||6 1/2 – 7||Excellent||Good|
|Pearl||3 1/2 – 4||Fair||Poor|
|Sapphire||9||Very Good||Very Good|
|Spinel||8||Very Good||Very Good|
|Tourmaline||7 – 7 1/2||Good||Very Good|
|Zircon||6 1/2 – 7 1/2||Poor||Fair|
How to Choose the Right Gemstone for the Job?
Now that we have learnt a little bit more about the durability of gemstones, you can begin to use this knowledge to choose the right stone for a piece of jewellery, whether it is for yourself, a stock piece or a bespoke commission for a customer.
When choosing a gemstone, there are two main considerations you need to make:
Now there are many different styles of jewellery out there but three of the most common are rings, earrings and pendants and from my experience, if you have to take more care of the stone when it is with you at the bench, there is a good chance that the gemstone isn’t really suitable for use in a ring because if it can’t take a couple of knocks at the bench, how well is it going to hold up being worn in a ring potentially day in day out?
Emeralds, Tanzanite and Opals are a great examples of this as they have to be treated with a lot of care due to not being very durable gemstones and this generally means that being worn in a ring, especially one that is worn everyday isn’t going to do the stone much good.
Instead, they are better suited to being used in a pendant or pair of earrings as their life is going to be much easier, with less chance of the stone getting broken or damaged.
For everyday rings such as engagement rings, a stone that is durable is going to be a much better option and one of the reasons why Diamonds are such a great choice.
If you have a customer who is wants to use a soft stone in an everyday ring, talking through and explaining the reasons why it is not the best option can save a lot of time, hassle and in some cases heartache (on both sides) but if they are determined to go ahead with their choice of stone, make sure that the paperwork they sign for the commission states that you have advised them against using this stone and that they understand the risks of choosing it, just helps cover your back!
The second consideration of which type of setting are you going to use? But knowing how durable a gemstone is, can have a big influence on the type of setting that you are going to choose as different styles of settings will give different levels of protection.
This is also useful if the customer is adamant of having a gemstone in a ring, that is not particularly durable.
For softer and more brittle gemstones, a rub over or bezel setting is often the best option. This is because not only does the setting provide more protection for the stone but the setting process can be more gentle.
If the gemstone is more durable, then more exposed settings such as claw or prong settings can be a great option as they don’t require as much protection from day to day knocks and bangs.
In this section, I will go through some scenarios where you will be using gemstones at the bench and give some tips on how to best care for your stones to prevent any unnecessary damage.
Ideally, stone setting is usually done once all of the metalwork is finished, this is because it prevents the stone from being accidentally damaged as most gemstones don’t deal too well with heat!
But there are times when a piece needs to be soldered and pickled when the stone is in place, such as repairs and this is where stability comes in to play as being exposed to the heat of a jewellers torch and/or being placed in a pickling solution can seriously damage the stone and this damage is often irreversible.
In most cases the best option is to remove the stone before doing any soldering as even very stable gemstones such as Diamonds can still be damaged when exposed to heat and most commonly the surface of the Diamond oxidises and the only way to repair this is to send it off and have it re-polished. Other coloured gemstones can break or it can cause discolouration on the stone.
But there are a couple of things you can do, the first is to use protective gels to insulate the stones from the heat and while they can do a good job, you still need to be aware that precious metals such as Gold and Silver are very good at conducting heat and this may get transmitted to the stone if the gel isn’t applied correctly.
One old school method is to immerse the stone/setting in water or wrap the stone and setting in a wet paper towel, this can be a good option as water does a very good job of pulling the heat out of the metal.
A more modern approach can be using laser welders as they don’t expose the stone to the same kind of conditions as soldering, unless you put the laser directly on the stone but try and avoid that if possible!
For lots of jewellers, stone setting is one of the scariest things to do as they are worried about breaking the stone. And as someone who does setting myself, I completely understand but knowing a little about the stone you are setting can make the process a little less stressful.
This is because when you know what the hardness and toughness of the stone is, you can adapt your setting style and I’ll give a couple of examples below:
Example 1: Say you are setting a stone in a bezel setting using a steel pusher and you slip and run the pusher across a facet on the stone. If the stone is harder than the pusher (tool steel is usually between 7-8 on the Mohs scale) such as a Diamond (10) or Sapphire (9), then the pusher should go along the surface of the facet and not scratch the stone.
Where as if the stone is the same hardness or softer than the pusher, such as an Amethyst (7) or Tanzanite (6), you will probably end up leaving a nice scratch on the stone and softer the stone, the more it will scratch.
Example 2: think of putting a stone in a claw (or prong) setting and the amount of pressure applied to the stone when the claws are being tightened. Stones with very good (Sapphire) or excellent (Jade) toughness are less likely to break when pressure is being applied when tightening the claw but it should be noted that ALL stones can break.
Where as stones with fair (Peridot) and poor (Emerald) toughness require much more care when being set as they are more likely to break when too much pressure is applied.
The final part of the jewellery making process is finishing and polishing and it is possible to damage the stone at this point, so care needs to be taken, especially when cleaning up the setting.
This is because abrasives, such as sand paper and rubber wheels can damage softer stones, harder stones such as Diamonds, Rubies and Sapphires are usually ok.
But on softer stones, these abrasives can take the finish off the stones and may leave a dull finish or if not careful or the complete removal of a facet from a stone (I learnt that one the hard way!). Ideally, you want to use an abrasive that is softer than the stone.
A similar issue can be found if you are using a very aggressive polishing compound as it can affect the surface of the gemstone. Once the gem is set, you don’t really want to use anything more aggressive than pre-polish (Tripoli, Luxi Blue etc) and then your finishing polish.
Once you have finished polishing, it is time to get the piece clean and sparkly but once again, care needs to be taken when doing this as Ultrasonics and Steam Cleaners aren’t suitable for all types of gemstones.
For example, with brittle stones such as Emeralds and Tanzanite, the ultrasonic waves can in some cases cause the stone to break, another side effect with Emeralds is that they are often fracture filled with oils that can be removed by the ultrasonic cleaner.
It is a similar story for steam cleaners and that is why both should only be used with durable gemstones such as Diamonds and Sapphires.
The best option in most cases is the classic, warm water, a small amount of washing up liquid, a soft bristled brush and a bit of elbow grease.
To learn lots about this, requires studying to become a gemmologist, like myself but if you don’t want to do this, there are some great books out there that you can use as reference guides at the bench and I’ll list a couple of these below:
Both of these contain information on the practical use of gemstones and are definitely worth adding to your collection.
While gemstones are beautiful, they do need to be treated with care when being used in jewellery and understanding the differences in durability between stones can make your life easier and also your customers lives easier.
But because of the variety of gemstones that are out there, it takes a bit of learning to know the difference between these gemstones but also using them at the bench will give you some first hand knowledge of what they are like to use and you will break stones when setting but this is all valuable experience for the next time you try to set another one.