Guide to buying diamond testers



See all diamond testers

This article is quite long, it is not necessary to read all of it, click on the sections in the CONTENTS that interest you. Alternatively, skip this article (with all its advice and recommendations) and go to our Diamond tester  comparison chart which simply lists each model and for what it can detect. 




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


We are unique in giving a very detailed appraisal of every product, telling you it’s features and its limitations. Every tester has its limitations, no matter what the make, no matter who you buy it from. This article is our guide to buying diamond testers. See also our guide to buying electronic gem testers. You might also be interested in specifically testing synthetic diamonds or specifically testing for Moissanite

The following explains the difference between the cheapest (they start at £12.50) and the most expensive (over £1000.00). 

FOR UNDER £50.00:

Example, ref. DT-DS-CRP

Example, ref. DT-DS

The cheapest (under £20.00) are not reliable. We do sell these and you may (as with any mail order purchase) return your purchase within 14 days if you don't like it, but after that you may not return it merely because many of the readings are incorrect. 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'.  

Be aware that there is a manmade stone called Moissanite which will read Diamond on these testers.

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.  

The 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.  


  • 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) but using the tester is a good starting point. 
  • you collect costume jewellery, you recognise costume jewellery, you just want to be sure that nothing reads 'diamond' on the tester. 

Conclusion: good for telling you if a stone is definitely not diamond / not Moissanite, not so good for telling you if a stone is diamond.

FOR ABOUT £50.00 TO £150.00. 

Technology: thermal conductivity.

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

Example, DT-EXP

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.

Conclusion: It will still (as above) 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

Technology: thermal conductivity (for diamond); electrical resistance (for Moissanite). Warning, some synthetic diamonds (not to be confused with non-diamonds!) read 'Moissanite'. Another warning: it is becoming increasingly difficult to test Moissanite, our manufactures check their production every few months and increase the sensitivity (as the manufacture of Moissanites gets more 'sophisticated').  It is expected that, soon, it will not be possible to distinguish Moissanite from diamond using electrical conductivity.

Example, MULTI-EXP

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 something decent, but you simply don't have the budget to spend £300.00 to £400.00 on testers for all the branches. 
  • 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

Technology: thermal conductivity (for diamond); electrical resistance (for Moissanite). Warning, some synthetic diamonds (not to be confused with non-diamonds!) read 'Moissanite'.

Example, MULTI-SAM

These test for diamond and Moissanite and are the most consistently reliable testers. Go for one in this price range 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 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 some synthetic diamond (not to be confused with non-diamonds!) reading 'Moissanite'.

Conclusion: in addition to its diamond-testing technology being the most reliable in extremes of temperature, it's best at detecting the latest (low-conductivity) Moissanites that other testers miss, though the disadvantage of making a tester this sensitive is that many synthetic diamonds (not to be confused with non-diamonds!) read 'Moissanite'.


Diamond can be natural crystals grown in the ground by nature (they take 2 to 3 billion years to grow) or synthetic diamonds 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 if 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". 


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. 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 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.


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.

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 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 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 '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, 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, 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.

Moissanite was first made in the 1990s, 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 diamond tester they will register diamond. 

Similarly, we used to say that antiques dealers would know if they had old (e.g. Victorian) items with 'old-cut' stones, and those stones would not be Moissanite because Moissanite hadn't been invented in Victorian times. However, Moissanite is now made to imitate even these.

To find out more about the history and characteristics of Moissanite, click here


Moissanite testers work by measuring electrical conductivity through the stone. Diamond is not electrically conductive, Moissanite is (though recently-made Moissanites are only very slightly conductive, you need a good quality tester). 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 older (and poor quality) diamond testers.  
2) it is now common to find Moissanite with extremely low electrical conductivity. Although recently-made testers are made extra-sensitive to detect these, the downside is that many synthetic diamonds show up as 'Moissanite'. 

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 (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).