Laboratory-grown diamonds that do not test as 'diamond'
by Julia Griffith FGA DGA EG
Laboratory-grown diamonds are diamond so test exactly the same as diamond, right? Um... not quite. There's a few differences when it comes to testing. The most bothersome is the test result of 'synthetic moissanite' on diamond testers.
Some laboratory-grown diamonds test as synthetic moissanite on diamond testers. This is specific to those produced by HPHT (high pressure, high temperature).
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This is a bit of a problem regarding identification. There is a risk that these stones may be misidentified. It can also be a confusing reality for consumers. Laboratory-grown diamond is the same material as diamond. So why aren't all laboratory-grown diamonds testing as diamond?
HPHT laboratory-grown diamond
The answer is quite simple. Despite being the same material, they can contain impurity elements uncommonly found in natural diamonds. The impurity concerned in this case is boron.
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Boron is an element that can become trapped within diamond's crystal structure. This is an extremely rare impurity in natural diamonds occurring less than 0.01% of the time. The presence of boron in diamond affects properties - namely the colour of the diamond (boron causes blue colour) and, important to this article, it alters diamond's electrical conductivity.
Without boron, diamond is an electrical insulator. With boron, diamond becomes a semi-conductor. The more boron present, the more intense the colour and the greater the electrical conductivity.
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There really needn't be much boron to have a significant impact on a diamond's properties. The Hope Diamond, the most famous blue diamond in the world, contains approximately 360 atoms of boron per billion atoms. The more boron there is within a diamond structure - the greater the impact on colour and electrical conductivity.
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Boron in HPHT laboratory-grown diamonds
Boron is present in many HPHT-grown laboratory-grown diamonds. A study by GIA in 2017 discovered that 70% of all the colourless HPHT laboratory-grown diamonds that came through GIA's testing lab contained boron.​
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If in high enough concentration (which is still low enough to be best counted per billion atoms), it can have a notable effect on the colour and electrical conductivity of the stone. These are the ones that most often test as 'synthetic moissanite' on basic diamond testers.
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Why does this occur?
A compound of boron is routinely used in the HPHT production process. This lowers the temperatures and pressures that would otherwise be required.
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This boron compound is placed within the reaction cell where the laboratory-grown diamonds grow. By default, it gets caught up in the forming crystals.
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Common colour tints in HPHT laboratory-grown diamond
The resulting stones
The HPHT-laboratory-grown diamonds that contain boron as an impurity can test as 'moissanite' on electrical probes and diamond multitesters (dual thermal and electrical probes).
These often have a blue tint, which is sometimes referred to as a 'blue nuance'. HPHT laboratory-grown diamonds with green and yellow tints can also test as 'synthetic moissanite' due to containing boron (amongst other impurities thus appearing a different colour to blue). These colour tints can be seen in laboratory-grown diamonds with a colour grade equivalent to G and below.
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Diamond multitester by GemTrue
The diamond testers
For clarity, the diamond testers we are discussing are the simple probe-like testing tools designed for separating diamond from diamond simulants (lookalikes of diamond). We are not discussing diamond verification tools to separate diamond and laboratory-grown diamond.
It's important to know that the diamond testers are working perfectly. It's the artificial stones that are responding differently to what is expected or differently to how they used to react.
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Diamond testers were created back in the 80s and 90s for a very important yet simple purpose: to separate diamond from diamond simulants. This is before the commercialisation of laboratory-grown diamonds so these were not a consideration.
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Cubic zirconia (CZ); a popular diamond simulant
The first diamond testers succeeded in this task through the use of a thermal conductivity test. At the time, all available diamond simulants had significantly lower thermal conductivity than diamond.
Diamond is a fantastic thermal conductor. In fact, they are the most effective heat conductor of all materials - at least 4x more conductive than copper. Thermal probes were the perfect way to separate diamond from all of it's simulants. These are the OG diamond testers and were all that was required in the 1980s.
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The first diamond testers succeeded in this task through the use of a thermal conductivity test. At the time, all available diamond simulants had significantly lower thermal conductivity than diamond.
Diamond is a fantastic thermal conductor. In fact, they are the most effective heat conductor of all materials - at least 4x more conductive than copper. Thermal probes were the perfect way to separate diamond from all of it's simulants. These are the OG diamond testers and were all that was required in the 1980s.
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The following decade, synthetic moissanite entered the jewellery market. These created a problem. Synthetic moissanite is a good conductor of heat. Though nowhere near as thermally conductive as diamond - it is significantly more conductive than all the other gemstones. These diamond simulants tested as 'diamond' on the basic themal testers and the trade now had a problem to solve. How can synthetic moissanite be separated from diamond?
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Luckily, these synthetic moissanites had different electrical conduction to diamond. Synthetic moissanite was an electrical conductor. Diamond was not. Electrical testing probes were created for use after the thermal probe. If a 'diamond' result was obtained with the thermal tester, the electrical probe would then be used to discover whether it was indeed diamond (electrical insulator) or synthetic moissanite (electrical conductor).
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This worked very well and multitesters were created that perform with the thermal test and electrical test simulatneously so that just one piece of equipment is required.
Laboratory-grown diamonds confuse testers
Diamond testers still work as they did. They test for thermal and electrical conductivity to help separate diamond from simulants.
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The narrative of laboratory-grown diamonds is that they are exactly the same as diamond and have all the same properties. This is not 100% accurate as their properties can be highly unusual in natural diamond and results obtained from certain tests can be unique to the method that produced the stones.
In summary - some laboratory-grown diamonds have differing electrical properties to what is typical for diamond due to containing boron. These get the result of 'synthetic moissanite' on basic diamond testers.
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Note: if diamond testers were redesigned to give the results: 'electrical insulator' and 'electrical conductor' - this would be an accurate test result! However, less user-friendly... this conflicts with the purpose of these testers being created in the first place - allowing non-gemmologists to identify gem materials when they don't really know how to (which, to be honest, was always a risky game anyway).
Synthetic moissanite: a convincing look-a-like of diamond
Synthetic moissanite that tests as 'diamond'
Laboratory-grown diamonds testing as synthetic moissanite are not the only problem. To add to the confusion, some modern synthetic moissanites test as 'diamond'.
In recent years, synthetic moissanites have been created that are more 'colourless' (less tinted). These can have lower electrical conductivity to what is usual. As a result, these stones test as 'diamond' on many electrical-based testers.
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Single refraction (SR) in diamond
What can we do?
Do not rely on diamond testers for separating diamond and synthetic moissanite. Though they can reliably separate all other simulants from diamond (if the tester is decent and used correctly), a result of 'diamond' or 'moissanite' must be verfied further.​
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The easiest way* to separate diamond and synthetic moissanite is visually. Diamond is singly refractive. Synthetic moissanite is strongly doubly refractive. Concluding whether a particular stone is singly or doubly refractive (after a diamond tester has be used to eliminate all other possible simulants) leads to an accurate identification.
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*Easy for a trained professional.
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Double refraction (DR) in synthetic moissanite
The amount of fire, the specific gravity, and even the refractive index (if you have the right kind of refractometer) can also help to identify diamond from it's simulants.
Remember: if you don't know how to identify a particular stone - don't. It's too risky a game and we are in a professional industry with money and reputation at risk. ​
A CVD laboratory-grown diamond grown by Lightbox
What about CVD laboratory-grown diamonds?
Laboratory-grown diamonds produced through CVD (chemical vapour deposition) synthesis are not a concern regarding this issue.
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Though it's very possible to grow a CVD laboratory-grown diamonds with impurities of boron (just add a little to the recipe, right?), there is no practical benefit in doing so. Therefore, colourless CVD laboratory-grown diamonds do not contain boron and test as 'diamond' on diamond testers.
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Wanna hear this again? Check out my YouTube video which shows you some funky animations to help cement your understanding. Click the subscribe box in the bottom right corner to subscribe to my YouTube channel.
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