Guide to buying diamond testers



See all diamond testers



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


The following explains the difference between the cheapest (£12.50) and the most expensive (£350.00) and everything in between. 

For £10.00 to about £15.00

OK if your mission is to find the very cheapest tester. You may still (as with all mail order items) return it in 14 days if you don't like it, but after that you may not return it, even if you are getting too many incorrect readings, even if you find the readings 'hesitant' or 'not consistent'. This is what you get for this price, irrespective of what sellers on eBay and Amazon tell you! 

Be ware that there is a manmade stone called Moissanite which will read Diamond on this tester.

diamond reading means:

  • diamond or Moissanite (you won't know which)
  • any large stone if the stone is cool, any small stone if the stone is cold
  • anything at all if you haven't set the sensitivity correctly

For about £20.00 to £50.00. 

OK for occasional use (eg at boot sales, markets, eBay).

Be ware that there is a manmade stone called Moissanite which will read Diamond on this tester.

diamond reading on these cheap-ish testers means the stone is:

  • diamond or Moissanite (you won't know which)
  • 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 can be 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 it might be diamond.

For about £50.00 to £150.00. 

The quality gets better.  It will still (as above) read diamond on Moissanite but the results will be quicker (about a second) and more decisive (the same every time) and more accurate (correct nearly all the time). 

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 to be reassured.

For about £200.00. 

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 store branches, you know you will have to spend about £200.00 to get something decent, but you simply don't have the budget to spend £300.00 to £400.00 for 'the best', especially when buying testers in bulk. 
  • 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. 

For about £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, you just don't see the point in going for second-best.
  • you travel around, you might be using this in very hot country, you might be using this in cool conditions, you need something that will keep working when it's quite warm or quite cold.
  • you want the tester most likely to give correct readings on 'difficult' and 'unusual' stones. Most people don't come across these but you buy and sell thousands of diamonds and have seen everything and have found stones that are 'fool' the average diamond tester. 

Recommendation: The MULTI-SAM is the very latest model and has one big feature. As the manufactured of Moissanite gets better and better, nothing is guaranteed, but this is the tester most likely to give correct readings on 'difficult' Moissanites (i.e. they won't show up as 'diamond'). So if your question is, "All I want to know is - which is the best one?" then the answer is: the MULTI-SAM.



Approx. price incl. VAT

Tests mounted stones [1]



Ruby / 


Diamond selector II, (DT-DS-crp)(worst quality)

£12.50 Shows up as diamond Reads ‘simulant’

Diamond selector II  (DT-DS)
Cheap but works most of the time)


Shows up as diamond

Reads ‘simulant’

Diamond Experior (was Diamond Plus) £79.99 Shows up as diamond Reads 'Other Stone'

Moissanite tester by GemVue (Moissanite Plus ) MOISS-4)


Multi tester. Experior by Gemtrue (MULTI-EXP)




SAM multi-tester (MULTI-SAM)


Reads ‘simulant’

Handheld presidium gem tester (GEMTEST-PGI)


Shows up as diamond

31 stones

Standard Presidium (GEMTEST-P2)


Shows up as diamond

17 stones

Presidium Duo gem tester (GEMTEST-DUO-II)




Digital refractometer by Presidium (REFRACT-DIGI-PRESID)


55 stones

Refractometer (optical not electronic) (Refract-L)


Hundreds of stones [4]

[1] All testers test loose stones but not all can test mounted (set in jewellery) stones.

[2] The probe / analogue meter will test mounted stones, the digital meter will only test loose stones.

[3] The probe / analogue meter: 17 stones; the digital meter: 40 stones.

[4] Skill and practice is required to use an optical refractometer, instructions are not included, see any textbook about gemmology…or the internet.  


Diamond can be natural crystals grown in the ground by nature (they take 2 to 3 billion years to grow) or synthetic diamond crystals are grown in a laboratory by man (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 cannot tell the difference by looking at them, you cannot tell the difference using any of the above testers. There is a specialist tester that can distinguish them (the ARI), scroll down.

Moissanite is a synthetic stone (grown in a laboratory by man), it registers ‘diamond’ on all simple diamond testers (see the chart above). It is not diamond. Read all about Moissanite.

Simulant simply means ‘not diamond’. We used to call it imitation but now we tend to say simulant.  

Simulant 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 by a ring on eBay, it's described as 'diamond'. It is not diamond it's a fake. 
  • You have a brooch set with white sapphire, your friends ask it it's diamond, you say no, it's white sapphire. It’s not a ‘fake’, it’s just white sapphire, that’s what it is, nobody is pretending it's diamond.

DO NOT CONFUSE "SIMULANT" and "FAKE" (see above) with "SYNTHETIC". 



Approx. price incl. VAT

Tests mounted stones?

Natural diamond

Synthetic diamond


Ari Synthetic Diamond tester by Presidium


Arete Synthetic Diamond tester by Gemtrue £995.00

Natural diamond is mined from the ground; synthetic diamond is grown in a laboratory by man (manmade, lab-created). They are the same.

  • They are the same chemically (carbon)
  • They are the same physically (same crystal structure, hardness, density)
  • They are the same optically (refractive index, reflectivity, dispersion)
  • They are the same visually (the best expert using the best microscope can’t tell them apart)

So far, the only test is to shine an ultra-violet laser through the stone and measure the change in light (UV fluorescence).
You must be quite certain that you are testing a diamond. First test it on a good quality diamond tester THEN test it to see if it’s natural or synthetic. For this reason, I don’t like calling a ‘synthetic diamond tester’ a ‘diamond tester’, many non-diamond stones could show up as ‘diamond’. This is why these testers are often described as diamond screeners rather than diamond testers. 


A true story, 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 look like (you can find out - search Google Images). 

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.

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. 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 with who he was dealing with and, as far as I know, the transaction was successful. Whether he made any money I 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.


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 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 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 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 one 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 "manmade" or "lab-grown" or "lab-created") 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.

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 any cheap diamond 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. 

* Moissanite is found either as tiny black crystals of Silicon Carbide or as tiny green platelets. They tend to be 1 or 2mm in size, too small to 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 contain 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 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  usually 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 into 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).