Guide to buying diamond testers

Last updated: November 6, 2024

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SUMMARY
FOUR TYPES OF TESTER
TECHNICAL TERMS / TERMINOLOGY
SYNTHETIC V. NATURAL DIAMOND
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
COMPARISON CHART FOR DIAMOND TESTERS

This article is quite long, you don't have to read all of it! Experienced jewellers and professional diamond dealers will know all the basic information, just click on the sections, above, in which you are interested. Alternatively, go to the end of this article for a comparison chart for all the testers.

SUMMARY

There are now some synthetic Moissanites that test as diamond, on most* diamond testers and many synthetic diamonds that test as Synthetic Moissanite. on most diamond testers. NOTE: Natural diamonds and synthetic diamonds are both real diamonds. Naturals are mined. Synthetics are manmade from crystals grown in laboratories to the same recipe found in nature.

* Electrical Conductivity and 'multi' testers, scroll down for details. Basically, testers in the price range £199.00 to £350.00.

THIS DOES NOT MEAN ELECTRONIC DIAMOND TESTERS ARE OF NO USE. It merely means you must be aware of what each type can, and can't, detect. This applies to all makes and models of all electronic diamond testers, irrespective of any claims made by the sellers.

All the testers listed below are handheld electronic models suitable for testing stones set in jewellery.

There are two larger (bench models) that will test any loose stone that has a perfectly polished, perfectly flat top (table): the digital refractometer and the digital readout of the duo tester. These two will distinguish diamond from Moissanite and will test for dozens of other stones too (see our article on Electronic Gem Testers) - they will not test stones mounted in jewellery.

There is also an optical device (polariscope) that distinguishes diamond from Moissanite, it works on most mounted stones but works best on loose stones (other optical gemstone testers can be found at Optical and Mechanical Gem Testers).

SYNTHETIC MOISSANITE TESTING AS DIAMOND

This is because the manufacturers of synthetic Moissanite keep tweaking the formula, and now there are some synthetic Moissanites that evade the tests and show up as diamond on the electrical conductivity test. One eminent gemmologist sums this up like this: “…the diamond testers are working perfectly, it's the Synthetic Moissanite that is responding differently to what is expected.”

Options:

Take the risk, most of the time these (the better models anyway!) will be correct. Note, there will be more and more “iffy” Moissanites coming onto the market as time goes by.
Take a smaller risk, expose the stone to UV light (scroll down to How to improve the accuracy).

DIAMONDS TESTING AS SYNTHETIC MOISSANITE

To make the testers better at testing the latest Moissanites, the manufacturers make the testers more sensitive. As a consequence most (70%) of synthetic diamonds and some (0.01%) of natural diamond now read Synthetic Moissanite.

Options:

Whenever you get a ‘Moissanite’ reading, be aware that the stone might be synthetic diamond (scroll down to technical terms). So how do you know? Test it on the newest type of tester, an Ari, Arete or OTi. - basically, testers in the price range £950.00 to £1,500.00).
Meanwhile, if (on the older-type testers) you get a Moissanite reading, tell the customer, “It’s either synthetic Moissanite or synthetic diamond.” Otherwise you will upset customers who have real diamonds but don’t understand the difference between naturals and synthetics.

Here is a list of the four types of technology, each has its advantages and its limitations.

FOUR TYPES OF TESTER

Diamond testers currently use four different types of technology. Although every tester 'tests for diamond', each has its advantages and disadvantages, no tester is "100% accurate". The following applies to all testers no matter who you buy it from and no matter what they promise you.

- 1. TECHNOLOGY: THERMAL CONDUCTIVITY

This tells you:

if a stone can’t possibly be diamond (be it natural or synthetic) or synthetic Moissanite.
The ‘thermal conductivity’ part of a good "multi" tester also tests for ruby/sapphire.
A reading of not diamond/Moissanite or or ruby/sapphire is conclusive, it is not diamond, no other test is necessary.

However, if the test registers diamond, it might be diamond or it might be synthetic Moissanite, there is no simply way of knowing. Why?? Because this technology (thermal conductivity) dates from before synthetic Moissanite existed.

On the internet you can find plenty of these advertised as testing diamond, Moissanite, jade and ruby - this simply isn't true.

- What you get for your money with a thermal conductivity tester:

The cheapest (£12.50) is not reliable. At £24.50 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, the guarantee is for breakdown only, you may not return these cheap testers merely because most of the readings are incorrect, you get what you pay for. If you do want to go for this 'basic' technology, get a decent model, e.g. £82.50.

Be aware that there is a manmade (synthetic) stone called Moissanite, it reads Diamond on all thermal conductivity testers. A diamond reading 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.

- Uses:

for the cheapest (under £50.00), for instance - you collect costume jewellery, you recognise costume jewellery, you just want to be sure that nothing reads 'diamond' on the tester.
for the best (closer to £100.00), for instance - you are experienced enough to see when a stone is 'probably' diamond or is 'almost certainly not' diamond and just need a clue (a starting-point) from an electronic tester.

In either case you must show 'interesting' stones to an experienced jewellery, gemmologist or diamond dealer.

- see the products.

- 2. TECHNOLOGY: ELECTRICAL CONDUCTIVITY ('MOISSANITE TESTER')

This distinguishes diamond from synthetic Moissanite but only works on stones that give a diamond reading on a thermal conductivity tester (scroll down for multi-testers, they use both methods together). This was once simple: synthetic Moissanite was electrically conductive, diamond not so. However, synthetic Moissanites are now made with very low electrically conductivity which results in many them giving a diamond reading. To detect most of these low-conductivity Moissanites, the tester manufacturers have been making the testers more and more sensitive, but doing this has an unintended consequence: now, most synthetic diamonds now give a Synthetic Moissanite reading, because synthetic diamonds are also very slightly electrically conductive.

- What you get for your money with an electrical conductivity tester:

good value if you already have a thermal conductivity tester, and want to test for synthetic Moissanite. Two models, MOISS2 £19.50 (basic) or MOISS-4 MOISS-4 £79.00.(better).

- Uses:

As above. Remember: you must check the stone on a thermal conductivity tester first, this cannot be used on its own.

HOW TO IMPROVE THE ACCURACY OF ELECTRICAL CONDUCTIVITY TESTERS:

Here are some options that will help avoid synthetic Moissanite testing as diamond:

Use the UV light built into the tester, this works much of the time. It also helps to use the extra UV light that can be found in the sun (test in a room brightly-lit with sunlight…if possible).
Use a small, but powerful, UV torch, e.g. model UV-1-3W £27.50. ADVANTAGE: it’s powerful and it’s small - small enough to fit in the case with the tester. DISADVANTAGES: you must work close to the light (because it’s small); you will either need a third hand tool or get someone to help you; you must take two or three tests with the light shining on the stone at different angles.
Use a large UV light, e.g. model UV-100 £32.50. ADVANTAGE: works instantly, first time every time (on our samples of ‘difficult’ Moissanites). It’s large enough that you can just lay it on the table (no help required) and powerful enough to work at any angle. DISADVANTAGES: it’s large, 20cm long with an 8cm head.

There will come a time when most synthetic Moissanites will read diamond on multi-testers, even with UV light.

COMBINED: 'MULTI' (2-IN-1) THERMAL CONDUCTIVITY + ELECTRICAL CONDUCTIVITY

These carry out two different tests in two or three seconds. The first uses thermal conductivity, to test for diamond or synthetic Moissanite (see above), the second test, a fraction of a second later, uses electrical conductivity to confirm which of the two (diamond or synthetic Moissanite) it is this (electrical conductivity) test that is subject to the limitations listed above.

- What you get for your money with a multi tester:

The most popular is the multi-Experior £199.50 (we sell a few hundred per year), no only does it distinguish diamond and Moissanite but it also tests for ruby/sapphire, a useful 'extra'. the Multi-Sam, £359.00 is the best as regards the thermal conductivity test, it works in extreme conditions (e.g. heat and cold).

In all cases, see the limitations listed above.

- Uses (multi-Experior):

you trade in jewellery and want to be reasonably certain that you have a diamond or reasonably certain that you have a Moissanite...subject to the limitations listed above.
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.

- Uses (multi-Sam):

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.

In all cases, see the limitations listed above.

- 3. TECHNOLOGY: UV ABSORPTION

This fires a needle of UV light into the stone and measures the response. It is designed to distinguish natural diamond, synthetic diamond and Moissanite. However, it will not distinguish these from the myriad of other gemstones, so you will need a thermal conductivity tester to see if the stone might be diamond or synthetic Moissanite. If you don't already have one, you can select one as your free gift when you buy the Arete, £995.00. Similar model (no free gift) Ari, £1149.00. Or the model with a thermal conductivity tester built in, Oti, £1399.00.

UV absorption has its limitations as to the size, cut and colour. The stone must be minimum 1.6mm diameter (approx. 0.06ct); modern brilliant cut and perfectly-cut in proportion (not accurate on shallow or old-cut stones); colour D to J (not accurate if there's the slightest hint of yellow).

- uses

Although expensive (especially if you started by thinking you could get a decent-quality diamond tester for under £20.00) this is the only way forward. Dealers who buy these will find that they have the edge over everyone else, they can correctly identify (and deal in) synthetic diamonds and they can avoid mistaking synthetic Moissanite from diamond.

COMBINED: 'MULTI' (2-IN-1) THERMAL CONDUCTIVITY + UV ABSORPTION

See the OTi for the best all-in-one tester. This type of tester is quite new there are certain to be new models (and new prices) during the course of 2025.

- 4. REFRACTIVE INDEX

These testers are for loose stones only, so skip this section if you need to test stones set in jewellery. These testers are listed in the article, Electronic Gem Testers.

Electronic testers: either the digital-meter part of the Presidium Duo tester (the other part of the 'duo' is thermal conductivity) or a dedicated digital refractometer. These will distinguish diamond and Moissanite, they also test for dozens of other gemstones, see the article about electronic gem testers.

Optical testers: these will not test for diamond and Moissanite (the readings for these are out of range) but just for the record: standard refractometer or high-resolution refractometer. Again, see the article about electronic gem testers for details. These test for many dozens of gemstones.

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) then mined, or synthetic crystals (which take a few weeks to grow). Synthetic diamonds are also known as lab-grown, lab-created or manmade. The 'rough' crystals are cut and polished for jewellery.

Natural diamonds and synthetic diamonds are the same, they are the same physically, they are the same optically, diamond experts can't always tell the difference, even when they examine them under a microscope. Natural diamonds are also know as mined.

Synthetic: made by man, copying the exact formula found in nature. In the case of diamonds, this is usually by growing the diamond crystals in a laboratory. Read all about synthetic diamonds. Synthetic diamonds are also known as manmade, lab-grown, or lab-created.

Synthetic Moissanite registers ‘diamond’ on all thermal conductivity diamond testers. It is not diamond. The Moissanite you find in jewellery is always synthetic. 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 ('paste') or plastic.

Imitation or fake? These terms have 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.
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 an imitation (simulant), 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

The following is just a summary, read more about synthetic diamonds.

'Natural' diamonds grow (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 a 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). 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 mostly green. 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 (a gem-quality version of 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.

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 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).

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 this:

UV light can give an indication of probability when comparing natural diamonds with synthetic diamonds. 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, 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), a paste (glass), a sapphire (a small synthetic sapphire will do) and (if necessary, see above) a UV torch. 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.

COMPARISON CHART FOR DIAMOND TESTERS

For details of the limitations of each type of technology, see the top half of this article.

TESTER	READINGS	NOTES
Type: Thermal conductivity Diamond Selector II (cheapest) Diamond Selector II (slightly better) Diamond-Experior (best)	Not diamond and not Moissanite	Test is conclusive (do not test on Arete or Ari, result could be misleading).
	Diamond	Could be diamond (natural or synthetic) or could be Moissanite (to be certain, check with Arete or Ari).
Type: Electrical conductivity Moiss2 (Basic) Moiss-4 (better)	Moissanite	These are sold as SYNTHETIC MOISSANITE TESTERS, they are for testing stones that read 'diamond' on a thermal conductivity tester, they will not identify diamond on their own. However, due to changes in the way synthetic Moissanites are now manufactured, these don't reliably identify Moissanites either. The accuracy can be greatly improved by shining a good UV torch on the stone during testing (see the first half of this article) - then a 'Diamond' reading means it's natural diamond, and a 'Moissanite' reading means it's either synthetic diamond or Moissanite .
Type: Thermal conductivity with Electrical conductivity Multi-Experior (most popular) Multi-sam (best but pricey)	Not diamond and not Moissanite	Thermal conductivity: test is conclusive (do not test on Arete or Ari, result could be misleading)
	Ruby / sapphire	Thermal conductivity: The Multi-Experior also tests for ruby/sapphire - test is conclusive (do not test on Arete or Ari, result could be misleading).
	Diamond	Electrical conductivity: could be natural diamond or could be very low conductivity Moissanite (to be certain, check with Arete or Ari).
	Moissanite	Electrical conductivity: could be Moissanite or could be synthetic diamond (to be certain, check with Arete or Ari).
Type: UV absorption Arete Ari	Test the stone on a thermal conductivity tester, if the stone reads diamond/Moissanite, use the Arete to see if the stone is: synthetic diamond (or natural Type IIb natural diamond) or natural diamond or Moissanite. * DIAMOND EXPERIOR available as a free gift when you buy an ARETE providing you select it at the time of ordering online (if you already have a thermal conductivity meter you will probably prefer to select a different free gift).
Type: Thermal conductivity with UV absorption OTi	All-in-1: Non-diamond, natural diamond, synthetic diamond and Moissanite, no other tester required.
*Important note:* natural diamonds are mined, synthetic diamond (manmade) are grown in a laboratory, both are ‘real’ diamonds (they are extremely difficult to tell apart). Moissanite is a synthetic (manmade only) stone, it is not diamond. For loose stones: all of the technologies / testers listed above are for testing stones mounted in jewellery. For unmounted (loose) stones, including diamond and Moissanite, see the article Electronic Gem Testers.