Diamond tester trouble shooting

Troubleshooting guide for diamond testers, moissanite testers and multi testers

How to fix any problems with Quicktest, Gemtrue, Diamond Selector, Multi testers and Moissanite testers.




The purpose of this page is to take you through every possible reason why you might be having problems with a diamond tester, Moissanite tester or Multi tester. This is not an instruction manual and does not refer to any particular model.

It is worth reading through everything below, I am confident that it will 'fix' 90% of problems. But any machine made by man can break down. If you have bought your tester from Quicktest and it is under guarantee, and if you are certain that this troubleshooting guide won't fix it, then please return it and we'll send you a new one. If you are reading this because you have a problem with a tester bought elsewhere, please contact the seller.


Please read the instruction sheet. Even experienced users forget what to do if they haven't used a tester for a few weeks, and instruction sheets do get lost. Go to our   downloads page, scroll down to INSTRUCTIONS FOR DIAMOND AND GEM TESTERS, find the instructions for your model of tester. 

The following will fix most problems:

If just keeps flashing and will not warm up; if the READY light will not come on and/or you hear a rapid bleeping; if the LED lights fade, flash on and off, go out or zip up and down; if all the lights come on and there's continuous buzz: change the battery. If the battery only lasts a few minutes, buy a good quality battery. About a quarter of problems are due to flat batteries. Buy a good quality alkaline battery such as Duracel, UCAR or Procell. They cost £5.00 or £6.00 instead of £2.00 or £3.00 for an ordinary battery but they do last many months. A battery marked HIGH POWER, HEAVY DUTY, SUPER or EXTRA may not be good enough!

Do not test the same stone repeatedly, until it becomes hot. A hot stones will not register 'diamond', even if it is. Inconsistent readings miraculously become consistent if you wait a minute or two for the stone to cool down.

The test-tip (probe) on the tester must be clean, especially if it hasn't been used for some time. Clean it by rubbing it (at right angles) on in a circular motion on an ordinary piece of paper. This is stated in the instruction manuals of most models, along with a diagram. On a diamond-only tester, the tester will become less sensitive if the tip is dirty, i.e. you'll have to turn the LED Lights (sensitivity) up higher than usual. With a multi-tester, both diamonds and Moissanite will sometimes read, "Other stone", though not necessarily on every test, which will give the impression the tester is 'not consistent' or 'erratic' or "not reading diamond on known diamonds". Cleaning the tip fixes these problems.

Clean the stone. With the better testers it is sufficient to wipe it with a clean cloth.But please do use a soft clean cloth, not your shirt sleeve or a used hanky, you will merely add a layer of grease to the stone. With old models / cheap models, you will need to clean the stone more thoroughly. Alcohol is best (use with tissue or a cotton wool bud) because it evaporates quickly and leaves no residue (we have bottles of isopropyl alcohol), or if you don't have alcohol, use water. Do not use a proprietary jewellery cleaner, some of them leave a coating on the stone which confuses the tester. With recent / good quality models it is sufficient to wipe the stone with a cloth.

Place the tip on the stone at right angles, if you have a model with a retractable tip, press hard enough for the tip to retract - otherwise it simply will not work. If you have a model with fixed (not retractable) tip, don't be afraid to press firmly to make good contact (providing you hold the tip at right angles it won't break) but don't press with all your might. In all cases you should get a result in 1 to 2 seconds. If not, remove the tip immediately, wait several seconds for the stone to cool down and try again.

The above will solve most problems.

Other basic tips:

Wait for the READY light to come on between tests, make sure your finger(s) are touching the metal plate(s) on the tester, make sure you can clearly see the tip and the centre of the stone, even if this means finding your reading glasses or a magnifier.

The cheaper models don't work if the stone is cold. For instance, they cannot be used for buying at outside venues in Northern Europe in the winter (the general rule is: if it's cold enough to see your breath, then it's far too cold to use a diamond tester). 


Rather than read the remainder of this page, here are three suggestions:

Get a friend or colleague to use the tester. Get two or three friends or colleagues to use the tester. As each one does this, either leave the room or keep your mouth firmly shut, don't give them any clues, let them work it out for themselves. Very often if it works when they use it, see what it is that they are doing different.

Check the stone. A customer complained that a known diamond (100% guaranteed diamond) would not register DIAMOND on a tester. We sent another tester - same problem. Strangely, it would not register DIAMOND on a diamond tester at a local jewellers (a totally different model of tester). Then a second jeweller expressed the opinion that it wasn't diamond. Now maybe (I didn't ask) the stone had been in the family for generations and was 'very valuable' - but I do suspect that it wasn't diamond.

A simple test (to check that your tester is working) is to find a friendly jeweller and ask to test some of his brand new guaranteed-diamond diamond jewellery (but if it's taken out of a hot shop window, wait two or three minutes for it to cool down to room temperature). If you are the friendly jeweller, simply test one from stock, but do be certain that you have picked a diamond.


If your tester works consistently (it gives the same reading on the same stone every time) and correctly (it accurately identifies stones) then you really don't need to know the principle on which it works. However, if you are having problems (and you have read the instructions and are certain you are using it correctly) - then a knowledge of the principle will help.

The Principle

Touch a stone gently against your lip. Plastics (and, to a lesser extent, glass) feel 'warm' whereas sapphires, rubies and diamonds feel icy cold. This 'relative coldness' is what the diamond tester is measuring, technical term thermal conductivity.

Diamonds have very high thermal conductivity, they 'draw' heat, they are used extensively in the electronics industry to 'remove' (conduct) heat (they use diamond-coated components not gemstones), it is the high rate at which diamonds conduct heat that makes them react on a diamond tester and which makes them feel 'cold-to-the-touch'.

Too cold?

If a stone is really cold, it will feel cold-to-the-touch even if it's not diamond, and will register as DIAMOND on a diamond tester. This is a problem if a customer brings you a stone that has been outside on a very cold day, or if you are buying at outdoor fairs and markets in the winter. Take the stones into the warm then wait a few minutes for them to reach room temperature.

Large stones tend to stay cold and take a long time to warm up, small stones warm up quickly.

Too hot?

If a stone is very hot, it will feel warm-to-the-touch even if it is diamond, and will not register DIAMOND on a diamond tester. This happens when a diamond is taken from a shop window where hot lights or sunshine has heated it. It can be a problem in a heat wave, as the temperature approaches 30 degrees. Generally, high temperatures are not a problem here in the UK / Northern Europe.

Very small stones heat up easily, large stone tend to stay cool, but we do get a spate of complaints during a heat wave when the temperature gets into the thirties.

The readings on a tester - getting the feel

"Relatively cold" stones give a high (and for large diamonds, lightning-fast) reaction on the LED segments.
Large diamonds react so quickly that the very first time you see the lights shoot up the scale, you jump.
By contrast, a large ruby or sapphire will move the LED segments up slowly, and other (very large) stones may or may not produce any reaction at all.
So if you are testing a large stone and there's a slight reaction with the lights, you know it cannot be diamond, even if the lights tip into the DIAMOND zone (test it again, turn the sensitivity right down).

"Relatively warm" (and also very small diamonds) give a low, and often sluggish, reaction.
A very tiny diamond on a hot day may only move one or two LED segments up.
By contrast, any other small stone will give no reaction whatsoever.
So if you are testing a really tiny stone and get a reaction, it's probably diamond (test it again, turn the sensitivity right up).

You therefore do not have to 'learn' the chart on the back of the tester, you will very quickly get the 'feel' of how it works after testing your first few stones.

Of course, if you prefer, ignore all of the above and refer to the chart on the back of the tester...but you will get the 'feel' of how it works after testing a few stones, you will soon get to know that a small stone will give little or no reaction and that the sensitivity must be turned right up; you will soon get to know that a large stone will, if it is diamond, give a lightning-fast reaction and that you must turn the sensitivity right down; you will stop worrying about whether the lights 'tip' into the DIAMOND zone or not.

The battery

We have had these returned with every conceivable type of 'wrong' battery, marked HIGH POWER or SUPER or EXTRA. Those bought from boot sales or from the various types of pound shop last between 5 and 10 minutes; similar cheap batteries bought in supermarkets or department stores can last for an hour or two. I do believe customers who say the battery was brand new and was bought from a reputable shop, or that they have tried a few different batteries - but these batteries are not suitable. Buy a good quality alkaline battery such as Duracel, UCAR or Procell. They cost about £5.00 or £6.00 instead of £2.00 or£3.00 for an ordinary battery but they do last many months.


Please start by cleaning the tip of the probe. A dirty tip will give incorrect readings. Hold it at right angles (i.e. straight down, at 90 degrees) on a clean piece of paper and draw a few '8' shapes. The abrasiveness of the paper will clean the tip. If that doesn't work...read on.

The Principle

It measures electrical conductivity (resistance). The current travels all the way from your hand, through the metal plates on the tester, through the stone, through the setting of the jewellery, to your other hand, all the way round your body, back to your first hand, and back into the tester.

Moissanite is electrically conductive, diamond (except for one very rare type, and black diamond) is not.

From this principle you can now see that if you 'break the circuit', the tester will not register 'Moissanite'. You can also see that many items are electrically conductive, not just Moissanite, for instance, your body, and many household objects, even your fridge or a moist finger.

What this means

The significance of all of this is twofold.

Firstly, only test items that are likely to be diamond, this does not include your fridge or your finger; a Moissanite reading from your fridge or finger does not mean the tester is broken, it means that you are supposed to know that they are not made of diamond. And only test items which show up as 'Diamond' on a diamond tester. If you do not have a diamond tester, then a Moissanite tester will be of no use to you. The purpose of a Moissanite tester is to know if a stone that tested as diamond really is diamond, or if it is Moissanite.

Secondly, if a circuit cannot be made, you will not get a valid reading, e.g. if you are wearing rubber gloves, if you are not touching the metal plates on the tester, if your hands are too dirty (or too dry!) to make contact on the metal plates on the tester, if you have artificial hips or artificial knees.

You can also see, from the principle that you can accidentally make a circuit and get a MOISSANITE reading: by touching the setting of the jewellery, OR by placing the probe tip very very close to the setting so that the electricity 'jumps' - which is why you can't test very tiny stones (e.g. about 1 point). A layer of grease (and some proprietary jewellery cleaners) is likelty to be electrically conductive, so that diamonds will register as MOISSANITE - the stone must be clean, wipe it carefully with a very clean cloth (or clean it with an alcohol).


The purpose of a Multi Tester is to show you if the stone is diamond or not diamond. It carries out the diamond test (see above) then, a fraction of a second later, it carries out the Moissanite test (see above) and its microchip combines both readings and calculates a response.

You will not get correct readings if, in that first split second, you remove-and-replace the probe or slide it across the stone or press hard then let go then press hard. It only works if you put the probe on the stone (in the centre of the stone) firmly - once. As with all of these testers, this is no problem if you can see what you are doing and you have a steady hand. If you can't see, please find your reading glasses or a magnifier. If you don't have a steady hand you will not be able to use a tester.

Diamonds read DIAMOND based on thermal conductivity, all the rules and conditions listed above apply.

OLDER MULTI-TESTERS (up to mid- 2016) give the following readings:

DIAMOND, if it's high thermal conductivity
OTHER STONE, if it's medium thermal conductivity, especially rubies and sapphires, plus anything really cold (which would give a 'slight' reaction on a standard row-of-LED-lights diamond tester)
NO REACTION, if it's low thermal thermal conductivity (those that never give a reading on a standard diamond tester) - it could be anything, but it's not diamond
BUT - it's purpose is to tell you if you have a diamond or not, not to tell you if you have a sapphire/ruby, Moissanite, paste etc.

THE LATEST (EXPERIOR) MULTI-TESTER is more finely-tuned, it gives the following readings:

Anything electrically conductive reads MOISSANITE - it's probably Moissanite, it could be another electrically conductive gemstone. All the rules about electrical conductivity, listed above, apply.


Whenever anyone asks if a tester is "100% guaranteed to identify all diamonds" I take a deep breath. There are always always always exceptions. And these apply to all brands of diamond / Moissanite / multi testers.

PLEASE remember that Moissanite shows up as DIAMOND on simple diamond-only diamond testers. So if you already have a diamond tester, buy a Moissanite tester, if you don't have a tester at all, go for a Multi Tester, it will detect diamond and distinguish diamond from Moissanite.

There are three types of diamond that are electrically conductive and will therefore show up as Moissanite on Moissanite testers and Multi testers.

Firstly, a very rare type of diamond (imaginatively named Type II). These are indistinguishable from other diamonds. It's merely a quirk of their chemical composition that they happen to be electrically conductive - which is why they register MOISSANITE on multi testers. These are rare, you might never come across one, but they do exist, they turn up regularly at gem laboratories, to settle disputes arising from the use of testers.

Secondly, some synthetic diamonds (genuine diamonds, but grown in a laboratory rather than dug up from the ground) can read 'diamond' and 'Moissanite' on different parts of the stone. 

Thirdly, black diamonds. They too are electrically conductive and register MOISSANITE, you cannot test black stones. 


Using a diamond tester / Moissanite tester / Multi tester really is not complicated, the instructions (our instructions) are quite simple. A good start is to read them again, line by line, and make sure you are not doing anything wrong. Then, if you still have problems, read this page.