Guide to buying diamond testers



See all diamond testers


Various Diamond Selector diamond testers, which are genuine?



DIAMOND TESTERS AND U.V. (Ultra Violet light)


FOR £20.00 to £40.00. 
OK for occasional use (e.g. at boot sales, markets, eBay). Be aware that there is a manmade stone called Moissanite which reads diamond on all low-cost testers.  So a diamond reading on these testers means the stone is:

  • diamond
  • Moissanite
  • any large stone if the stone is very cold
  • anything at all if you haven't set the sensitivity correctly.  

Despite being somewhat 'basic', combined with a little knowledge these testers are useful. For instance:

  • you buy a ring from a jewellers, 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.
  • you collect costume jewellery, you don't expect any of it to be diamond but you have to check - it might be!
  • at a boot sale 99% of the jewellery is not diamond, your aim is find something (at a non-diamond price) that the tester indicates is probably diamond.

Recommendations: either the Chinese copy-Diamond Selector DT-DS or the original Japanese Diamond Selector DT-CULTI. These two testers look exactly the same, one is reliable, one is...not so reliable. See my guide to fake diamond testers.

If you also want to test for Moissanite, the MOISS-4 will check stones that have shown up as DIAMOND on the diamond tester, to see if they are Moissanite. The downside is that you have to use two testers, the upside is that both testers together will cost less than £100.00.

FOR ABOUT £200.00. 
the MULTI-EXP has proved so reliable and has so many good reviews that we now sell only this as a 'mid-range' model; we call it a "multi" tester because it tests for

  • diamond
  • Moissanite
  • ruby
  • sapphire

This is by the far the most popular, we sell a few hundred per year. This is the one to buy if:

  • you trade in jewellery and want to be reasonably certain that you have a diamond and you must be able to distinguish diamond from Moissanite. And it's handy to be able to test for rubies and sapphires too.
  • you buy equipment for several branches, you know you will have to spend about £200.00 to get something good, but you simply don't have the budget to spend £300.00 to £400.00 for 'the best'. 
  • you are an expert in diamonds, you don't really need a diamond tester, but you want to show your customers that you are selling them diamonds - and you really don't want to get any ambiguous readings! 

 FOR OVER £300.00 to £400.00

These test for diamond and Moissanite and are the most consistency reliable testers. Go for one of these if:

  • you are a professional dealer, you make your living dealing in high-value jewellery, I just don't see the point in going for second-best.
  • jewellery is just a sideline but you would still like the best diamond tester available.

Recommendations: the MULTI-PRESID is the well-established Presidium model, it's so good that it's regarded as the 'benchmark' for top-rate testers. The MULTI-SAM is the very latest model and has one big feature - some Moissanite (that manmade stone) can now be made so that it fools even the sophisticated multi-testers...but not this one. So if your question is, "All I want to know is - which is the best one?" then the answer is: the MULTI-SAM.


A true story, the tale of a man who was fooled

We sold a diamond tester to a man who went to Africa to buy 'bargain' diamonds (uncut 'rough' diamonds) from a 'contact'. After a very long time 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 the 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 the diamonds 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, and if presented with huge stones that look 'too watery' or 'too sparkly' he will be suspicious, even before turning on the diamond tester. But very few people know what diamond crystals look like. If you are planning on parting with thousands of pounds, do some research, e.g. search Google images for rough diamond crystals to see what diamond crystals should look like.

There are other precautions you can take if you are spending large amounts of money and know nothing about diamonds.

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.

Tricksters have been known to 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. I 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 about who he was dealing with and, as far as I know, the transaction was successful. Whether he made a profit I have no idea.


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 a diamond tester, the results on a Moissanite tester were inconclusive, it depended on where, on the stone, the probe was placed.

Even without a gemologist spending a few hours examining and testing the stone, 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.

Gemmologist's conclusion: it was not diamond, it was Moissanite.

My conclusion: electronic testers are not suitable for use on black stones.

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.  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 probably not diamond; if some fluoresce and some don't...then they might be diamond...or they might not.

The information in this paragraph is for the gemmologist or professional diamond dealer, the average jeweller or antiques dealer does not need to know any of this.

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 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 a diamond under a UV light tells you nothing. 

UV light can also be useful when grading a diamond for colour, because white diamonds that fluoresce under UV light also fluoresce under UV light present in daylight, and this can make the stone appear a better colour than it really is. Again, this information is for the gemmologist or professional diamond dealer, the average jeweller or antiques dealer does not need to know this.

Explanation: 'Synthetic' (also known as "lab-grown") does NOT mean 'imitation. A synthetic stone is 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, this requires a very special "synthetic diamond tester". 


Moissanite (read all about it) is a manmade stone (it doesn't exist in nature in a form that can be cut into gemstones*). It was only 'invented' in 1998, it is of no great value (though it does have a 'value' due to clever marketing), its only significance is that is 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.

* It's found either as tiny black crystals of Silicon Carbide or as tiny green platelets. You can see pictures of Moissanite crystals here, they are less than 1mm in size and cannot be cut into gemstones. Part of the publicity about Moissanite is true: that the French chemist Henri Moissan (hence "Moissanite") discovered the mineral in a meteorite crater in Arizona in 1893. But the gemstone stems from a 1998 patent for "translucent silicon carbide of a single polytype that are grown in a furnace sublimation system" (i.e. it is grown in a furnace, it does not exist in nature) - and you will notice that, for the sake of the patent, it is known as "Silicon carbide gemstones" rather than the more romantic name "Moissanite". You've probably heard of Silicon Carbide under another name, it's a black grit used as the abrasive carborundum.


Moissanite testers work by measuring electrical conductivity through the stone. Diamond is not electrically conductive, Moissanite is. However, there is a very rare type of diamond (Type II diamond) which has an unusual chemical composition (they do not contain nitrogen, they contains boron) and this makes the diamond electrically conductive, i.e. they register 'Moissanite' on both Moissanite testers and on 'multi' diamond testers. 

We had one of these diamonds and discovered that (with this particular diamond) some parts of the stone registered as Moissanite and some parts registered as Diamond - it turned out to be a synthetic (lab-grown) diamond. 

If you are not a diamond dealer handling thousands of diamonds, it's unlikely that you will ever see one of these. But, rare as these diamonds are, they are turning up at gem laboratories, sent in by anxious retailers following complaints by their customers (who have bought an expensive diamond only to find it registers 'Moissanite' on the tester).


Unlike diamond testing, UV light does make a difference when testing a Moissanite on a Moissanite tester or on a combination (multi) tester.

The discovery was made by a gemmologist who worked out that, according to the laws of quantum physics, UV light should make a difference to electrical conductivity. He then took a deep breath and set about finding out just which exact wavelengths of UV light were required. To his amazement he found that any UV light worked. Moissanite testers that don't work on some 'difficult' Moissanites (rare as they are) work perfectly when the stone is exposed to UV light.

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 in to the tester; our Multi Experior is the only model that will enable you to shine the UV light on the stone whilst testing (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).


A cheap diamond tester (a diamond-only tester) will distinguish diamond from non-diamond with the exception of a manmade stone called Moissanite, which will register 'diamond' - read all about Moissanite 

A multi tester will distinguish Moissanite from diamond. 

I used to say that Moissanites weren't common and that you were unlikely to come across them. They are now quite common. They have now been around for such a long time (they were first made in the 1990s) that they are being passed down to the next generation, who may have no idea what the stones are. If you test them on a cheap (diamond-only) tester they will register diamond. 

Similarly, I used to say that antiques dealers would know if they had a ring with 'old-cut' stones, and those stones would not be Moissanite . However, Moissanite is now made to imitate even these.

For both these reasons it's even more important to use a tester that can distinguish Moissanite from diamond. 

This does not mean that the cheap (diamond-only) testers are of no use. They are fine if you want to know that a stone cannot possibly be diamond; they are OK if you want to know that a stone is "probably" diamond, then get it tested properly.