Guide to buying diamond testers
CONTENTS
BUYING GUIDE, SUMMARY
THE TECHNOLOGY and WHAT YOU GET FOR YOUR MONEY
THRE THREE TYPES OF TECHNOLOGY
TECHNICAL TERMS / TERMINOLOGY
SYNTHETIC V. NATURAL DIAMOND, SUMMARY
MOISSANITE
DIAMOND TESTERS AND U.V. (Ultra Violet light)
A WARNING. THE TALE OF A MAN WHO WAS SCAMMED.
BEWARE OF UNUSUAL STONES...AND ALWAYS BE AWARE OF THE OBVIOUS
SEE ALSO...
BUYING GUIDE, SUMMARY
This article is quite long, you don't have to read all of it!
Essential information for everyone:
- what is the difference between a £12.50 and a £1,300.00 tester? What do you get for the money? And: there are three types of technology, what are they and what are the limitations of each?
Additional information (professional jewellers and diamond dealers will know this already):
- Buying diamonds abroad and avoiding scams on eBay etc.
- What's the difference between natural and synthetic diamond?
- What is Moissanite? What is a 'simulant' and what is a 'fake'?
See also Testing for Synthetic Diamonds if you are specifically interested in synthetic diamonds; Distinguishing Diamond from Moissanite if you want to know more about Moissanite; or Electronic Gem Testers (for the general testing of gemstones rather than diamond).
THE TECHNOLOGY and WHAT YOU GET FOR YOUR MONEY
FOR UNDER £50.00:
Technology: thermal conductivity
(Scroll down to see the limitations of thermal conductivity testers)
The cheapest (under £20.00) are not reliable. At £20.00 to £50.00 the reliability is definitely better, but these are still 'cheap' diamond testers and you still 'get what you pay for'. You may return any item within 14 days if you don't like it, but after that you may not return these cheap testers merely because many of the readings are incorrect, you get what you pay for.
Be aware that there is a manmade stone called Moissanite which reads Diamond on these testers.
A diamond reading on these cheap testers means the stone is:
- diamond or Moissanite (you won't know which)
- a large stone if the stone is cold
- any stone at all if you haven't set the sensitivity correctly.
These cheap testers are useful if all you want to know is if the stone definitely isn't diamond and isn't Moissanite. For any other reading you are advised to get the opinion of a gemmologist, diamond dealer or experienced jeweller.
Uses:
- you buy a ring online, the receipt says it's diamond, the diamond tester says it's diamond - it's looking good...or if the diamond tester says it's definitely not diamond - it's looking bad. Whatever the reading on the tester, you will still need to get a second opinion (from a human expert).
- you collect costume jewellery, you recognise costume jewellery, you just want to be sure that nothing reads 'diamond' on the tester.
FOR ABOUT £50.00 TO £150.00.
Technology: thermal conductivity
(Scroll down to see the limitations of thermal conductivity testers)
Be aware that there is a manmade stone called Moissanite which will read Diamond on this type of tester.
Technology: thermal conductivity
(Scroll down to see the limitations of thermal conductivity testers)
Recommended if:
- you are not a full time professional trader but you do buy at fairs / markets / boot sales and sell on eBay (or vice-versa) and need something more reliable than a 'cheap' model.
- you do have a good idea as to when a stone is 'probably' diamond or is 'almost certainly not' diamond and just need a clue (a starting-point) from an electronic tester.
It will still (as with the cheap testers) read diamond on Moissanite but the results will be quicker (about a second) and more consistent (the same every time) and more accurate (correct most of the time) than the cheap models. You will still need to take 'interesting' stones to a, gemmologist, diamond dealer or experienced jeweller to be certain, but at least you know you are using a reliable thermal conductivity tester.
Note: this is a good choice when used alongside a specialist synthetic-diamond tester (about £1000.00). The two testers together give the most reliable results of all.
FOR ABOUT £200.00 to £350.00
Technology: thermal conductivity (for diamond) and electrical resistance (for Moissanite). Scroll down to see the limitations of thermal/electrical conductivity testers.
Tests for:
- diamond
- Moissanite
- ruby / sapphire
This is by the far the most popular, we sell a few hundred per year. Go for one in this price range if:
- you trade in jewellery and want to be reasonably certain that you have a diamond or reasonably certain that you have a Moissanite. And it's handy to be able to test for rubies and sapphires too.
- you buy equipment for several store branches, you know you will have to spend about £200.00 to get anything decent, but you simply don't have the budget to spend £300.00 or £400.00 on each branch.
- you are an expert in diamonds, you really don't need a diamond tester, but you want to show your customers that you are selling them diamonds - and don't want to risk getting ambiguous readings.
Technology: thermal conductivity (for diamond) and electrical resistance (for Moissanite). Scroll down to see the limitations of electrical conductivity testers.
Tests for:
- diamond
- Moissanite
This tester for diamond and Moissanite is the most consistent and reliable. Go for this one if:
- you are a professional dealer, you make your living dealing in high-value jewellery, you just don't see the point in going for second-best.
- you'd like something that work well in the widest range of temperatures, e.g. in the summer in a heatwave or outside in the winter in the cool (not freezing!) especially if you're going to be using it in a hot country.
- you want the most sensitive tester for giving correct readings on 'difficult' Moissanites - though making a tester this sensitive does result in most synthetic diamond (not to be confused with non-diamonds!) reading 'Moissanite'.
For about £1000.00
Technology: UV absorption. Scroll down to see the limitations of UV absorption testers.
These distinguish natural diamond from synthetic diamond, and also test for Moissanite. However, you must be certain the stone is one of the above (natural diamond, synthetic diamond or Moissanite) - which means testing the stone on a thermal conductivity or electrical conductivity tester first (see above).
Ari by Presidium. Smallest, neatest, best-known manufacturer.
Arete by GemTrue. Easiest to use, and if you don't already have a diamond tester you can select one as your free gift when you buy the Arete.
These are reviewed in detail in our article about testing for synthetic diamonds.
FOR ABOUT £1,399.00
We now come the logical question, why so complicated? Can't someone just combined the various technologies and make a diamond tester that 'works'.
They have. Though, at the moment, they are new and quite expensive: a single tester that tests for natural diamond, synthetic diamond, and non-diamond including Moissanite. I would guess that prices will come down over the course of 2025.
OTi by Presidium.
THRE THREE TYPES OF TECHNOLOGY
There are three types of technology for electronic diamond testers. Although you don't need to know how they work, it is important to know the limitations of each type. An analogy: you are buying a car, you decide you don't need to know the basic differences (petrol/diesel, hybrid, electric), you just buy the car which has the most features for the price. Then you find it's electric and you don't have a charging point.
The original (simple) diamond testers worked on thermal conductivity, this tells you if a stone cannot possibly be diamond (be it natural or synthetic) or Moissanite. This test is conclusive, no other test is necessary). If the test registers diamond, it might be diamond but it might be synthetic Moissanite.
Multi testers carry out two totally different tests in just two or three seconds. The first test uses thermal conductivity (see above). The second test (electrical conductivity), a fraction of a second later, tests for Synthetic Moissanite. This was once simple: Synthetic Moissanite was electrically conductive, diamond not so. However, Moissanites are now manufactured with very low electrically conductivity which results in many them giving a diamond reading. In response, the tester manufacturers make their testers more sensitive, but doing this has an unintended consequence: synthetic diamonds (which are also very slightly electrically conductive) reading Synthetic Moissanite.
The new type of tester (Arete or Ari) works on UV absorption and will distinguish natural diamond from synthetic diamond and also detect Synthetic Moissanite. However, they will not distinguish these from non diamond (imitation / simulant). So you will need to check on a thermal conductivity tester first.
The above applies to any tester you buy from anyone, it applies no matter what the seller promises you!
TECHNICAL TERMS / TERMINOLOGY
Natural: made by nature; in the case of gem material, dug up out of the ground (mined) usually in the form of crystals which are then cut into gemstones.
Diamond can be natural crystals grown in the ground by nature (they take 2 to 3 billion years to grow) or synthetic (which takes a few weeks). Synthetic diamonds are also known as lab-grown, lab-created or manmade.
Natural diamonds and synthetic diamonds are the same, they are the same physically, they are the same optically, diamond experts can't always not tell the difference by examining them them under a microscope.
Synthetic: made by man, copying the exact formula found in nature. In the case of gemstones, crystals are grown in a laboratory then cut into gemstones.
Synthetic Moissanite registers ‘diamond’ on all simple diamond testers. It is not diamond. Read all about Moissanite.
Simulant / imitation simply means ‘not diamond’. It might look like (simulate, imitate) diamond but it isn't.
Artificial isn't really a 'technical' term. It could mean anything manmade; it could mean 'assembled' (more than one material, glued or mixed together); it is often taken to mean glass or plastic.
Imitation or fake? This has nothing to do with gemstones, it depends on the circumstances:
- You are looking for diamonds, you are not interested in anything else, ‘simulant’ means you think it looks like (simulates) diamond but it isn't, it's "non-diamond".
- You buy a ring on eBay, it's described as 'diamond', you've paid for diamond. It is not diamond it's a fake.
- You have a brooch set with a white sapphire, your friends ask if it's diamond, you say no, it's white sapphire. It’s not a ‘fake’ and it's not a 'simulant' because you never thought it was diamond - it’s just white sapphire, because that’s what it is.
DO NOT CONFUSE "SIMULANT" and "FAKE" with "SYNTHETIC".
"SYNTHETIC" HAS A VERY SPECIFIC SCIENTIFIC MEANING (see above).
SYNTHETIC V. NATURAL DIAMOND, SUMMARY
A 'natural' diamond grows (as crystals) in the ground. Also known as 'mined'.
'Synthetic' (also known as "manmade" or "lab-grown" or "lab-created") does NOT mean 'simulant ' (imitation). 'Synthetic' means it's grown in the laboratory to the same recipe found in nature, and the aim of the manufacturer is to make an end-product which is identical to its natural counterpart. Synthetic diamonds are diamonds (unlike, for instance, Cubic Zirconia or Moissanite which are not diamonds) - and most synthetic diamonds register DIAMOND on diamond testers...because they are diamond. It is extremely difficult to tell the difference between natural and synthetic diamonds, that is why the latest testers are so expensive.
MOISSANITE
Synthetic Moissanite (read all about it) is a manmade stone. Although it exists in nature, most Moissanite crystals are 1mm to 3mm across, and any that are large enough to be cut into gemstones are extremely rare. Synthetic Moissanite was 'invented' in 1998, it is of no great value, its only significance is that it registers 'diamond' on diamond testers. Moissanite does, to the non-expert, look remarkably like diamond - but it is not diamond it is Moissanite, i.e. Moissanite is not a 'type' of diamond, it is another stone altogether (silicon carbide).
Moissanite was first made in 1998, that's long enough ago that they are now being passed down to the next generation, who may have no idea what the stones are. If you test them on any cheap (£20.00 to £100.00) diamond tester they will register diamond and, increasingly, there are Moissanites that register diamond on mid-priced testers (£150.00 to £350.00).
Similarly, we used to say that antiques dealers would know if they had an old (e.g. Victorian old-cut) stone - which therefore couldn't be Moissanite. However, Moissanite is now made to imitate even these.
To find out more about the history and characteristics of Moissanite, click here.
MOISSANITE TESTERS, TYPE II DIAMONDS AND SYNTHETIC DIAMONDS
Synthetic Moissanite testers work by measuring electrical conductivity through the stone. Diamond is not electrically conductive, synthetic Moissanite is (though recently-made synthetic Moissanites are only very very slightly conductive, you need a good quality tester and even these, nowadays, won't detect all Moissanite). However:
1) there is a very rare type of diamond (Type II diamond) which has an unusual chemical composition (they contain boron) and this makes them electrically conductive, i.e. they register 'Moissanite' on most diamond testers.
2) it is now common to find Moissanite with extremely low electrical conductivity. Recently-made testers are made extra-sensitive to detect these, the downside is that many synthetic diamonds show up as 'Moissanite'.
MOISSANITE TESTERS AND UV (Ultra Violet light)
Unlike diamond testing, UV light does make a difference when testing a Moissanite using the electrical conductivity technology (see above). .
If you are buying a Moissanite tester, buy a UV light too, they really are not expensive (see how UV light works). If you are buying a combination diamond/Moissanite (multi) tester, there is a UV light built into the tester; the Multi Experior and Ultratester 3+ are the only models that will enable you to shine the UV light on the stone whilst testing (most other models give you the choice of testing the stone or using the UV light, but not both at the same time, which isn't really of any use).
DIAMOND TESTERS AND U.V. (Ultra Violet light)
Some models of diamond tester have a built-in UV light, and this has led to the quite logical assumption that UV light can be used for testing diamonds. This is not true. There is absolutely no way you can distinguish diamond from non-diamond using UV light. The only useful thing to know is that diamonds fluoresce (glow) randomly under UV light. So if you have a cluster ring or a diamond brooch and all the stones react in exactly the same way (whether they fluoresce or not) - they are unlikely to be diamond; if some fluoresce and some don't...then they might be diamond...or they might not.
The very technical information in the following paragraph is for the gemmologist or professional diamond dealer, the average jeweller or antiques dealer does not need to know any of the following:
UV light can give an indication of probability when comparing natural diamonds with synthetic diamonds (see 'Explanation' at the bottom of this section). For examining 'parcels' of diamonds (e.g. mele) you will need a UV lamp that provides long-wave AND a short-wave UV light. View the stones on a black background in a completely dark room (or a viewing cabinet). Natural colourless diamonds (of which about 40% fluoresce) usually fluoresce more under long-wave than under short-wave; some synthetic diamonds have just the reverse reaction, the fluorescence is weak (or nil) under long-wave, and strong (or stronger) under short-wave. So if you buy loose diamonds, check each parcel under UV light to judge the probable mix of 'naturals' and 'synthetics'. But examining one diamond under a UV light tells you nothing.
UV light should not be used for colour-grading because white diamonds that fluoresce under UV light appear a better colour than expected. Again, this information is for the gemmologist or professional diamond dealer, the average jeweller or antiques dealer does not need to know this.
A WARNING. THE TALE OF A MAN WHO WAS SCAMMED.
We sold a diamond tester to a man who went to Africa to buy 'bargain' diamonds (uncut 'rough' diamond crystals) from a 'contact'. After much haggling they agreed a price (I think it was $50,000.00) he tested the stones, they registered DIAMOND on the tester, they were sealed in a container and he signed across the seals. The money was transferred into the seller's bank account and the following day the buyer collected the stones. The seals on the container were still intact, he brought them back to England.
Guess what? When he got them back to England he found they weren't diamonds at all. The customer rushed them over to us, we tested them on five different diamond testers - they were not diamonds.
This, in a way, was a clever 'con' - because the average jeweller will know what a cut diamond looks like, if they look 'too watery' or 'too sparkly' he will be suspicious, even if he doesn't quite know why they look 'wrong'. But very few jewellers know what diamond crystals ("rough") look like (you can find out - search Google Images).
There are other precautions you can take if you are spending large amounts of money.
Firstly, if the deal seems too good to be true, it's because it is too good to be true, "Cheap Diamonds" are like "cheap gold" or "cheap cash" - they simply don't exist...unless you become involved with criminal gangs. These gangs smuggle real diamonds, drugs and people, and you really do not want to find yourself in a remote part of the world, escorted by armed men to remote mines, then pressurised into parting with money.
Do not let that diamond tester out of your sight, even for a minute. They can be tampered with (by re-soldering wires inside) so that everything reads DIAMOND. Our man in Africa managed to return to England without the diamond tester, it had been "mislaid in the confusion" - what a surprise!
Keep on your person a genuine diamond (it need not be large) and a paste (glass) and a sapphire (a small synthetic sapphire will do) and test each of them before testing your purchases. You will then know if the diamond tester has been tampered with.
It is also possible, with any machine made by man, that the machine develops a fault - so use those three stones to check the machine.
I've heard that tricksters can store stones in ice to cool them so that the diamond tester falsely reads DIAMOND. Try touching the stone gently against your upper lip to see if it feels icy cold, try clasping it in your warm hand and chatting to the sellers for five minutes. If they become agitated it might be because the stone is rapidly reaching room temperature and is about to register NOT DIAMOND on your tester.
These precautions do not apply to everyday dealing where the amounts of money involved are relatively small, there is no need to become paranoid. And it can happen that buying diamonds directly from the mines works out OK. We did sell a diamond tester to someone who was going to Africa but who had recommendations from friends who lived there, and he was happy with who he was dealing with and, as far as I know, he did buy diamonds. Whether he made any money we have no idea.
One final warning about buying diamonds from overseas. When you bring them back, customs will want to see the Kimberly Certificate to show where they originated (to prevent diamonds being used to finance war). In the UK, if you don't have the correct paperwork, the diamonds will be confiscated; in the US you will be arrested.
BEWARE OF UNUSUAL STONES...AND ALWAYS BE AWARE OF THE OBVIOUS
One stone that caused prolonged correspondence on an internet forum for gemmologists, was an unknown stone bought as 'black diamond'. It was opaque, so the usual examination of inclusions, under a microscope, was not possible; the surface was 'pitted' like granite; the SG (specific gravity) wasn't quite right; and although it certainly registered 'diamond' on diamond testers, the results on a Moissanite tester were inconclusive, it depended on where, on the stone, the probe was placed.
There were two causes for concern. Firstly, it was bought by a serviceman in Afghanistan and most stones sold to servicemen in this area were not diamond. Secondly, 'proof' that it was natural black diamond came in the form of a lab report (certificate) from New Delhi, from "an ISO 9001:2008 Company comprising GIA alumni." A certificate is worthless unless you can check that the company (and the certificate) is genuine - and even then, you need the skill to match the stone with the certificate.
Conclusion: it was not diamond, it was Moissanite.
The lesson: electronic testers are not suitable for testing black stones.
SEE ALSO...
See also Testing for Synthetic Diamonds if you are specifically interested in synthetic diamonds; Distinguishing Diamond from Moissanite if you want to know more about Moissanite; or Electronic Gem Testers (for the general testing of gemstones but not so good with diamond).