Testers & scales calibrated before despatch

Gem & precious metal testers, since 1986

Diamond tester Instruction manuals

Most DIAMOND SELECTOR are fake - details.

Instruction manuals for diamond testers, combination testers, multi testers, diamond-Moissanite testers.



MODEL DT (marked DT on back of tester)

MODEL DT (marked Thermal Tester)

TROUBLESHOOTING (all diamond testers)






Supplied with PP3 battery. To change battery, slide battery cover in direction of arrow, pull old battery out gently, do not keep pulling until you pull the wires out. The stone holder is for loose (un-mounted) stones, since you would not be able to hold them in your fingers.

This model is particularly good for coping with extremes of temperature. Unlike other models, it has been calibrated for Northern Europe where outdoor markets and unheated halls can be cold. However, avoid extremes of temperature between the stone and the tester. For instance, If you are working in the warm and a customer brings in a stone that has been stored in the freezing cold, or if you take a stone from a hot window display. Wait five minutes for the stone and tester to reach the same temperature.


Turn the SENSITIVITY knob just enough for it to click. The BATTERY OK light will light up and the segments will flash briefly. Then wait for the READY light (20 to 30 seconds).

Gently remove the rubber protective cap from test tip (remember to replace it after use).

Turn the SENSITIVITY knob until some of the segments are lit (usually the four red segments, see SETTING THE SENSITIVITY below).

Press test tip firmly onto the top of the diamond at right angles. Do not attempt to scratch the stone or you will break or bend the test-tip. Make sure you are touching (with your hand) the metal plate on the back, then if you miss the stone and touch the setting by accident an alarm will sound.

Lights remain static (nothing happens) = not diamond

Lights up slowly, doesn't register 'diamond' = not diamond(probably ruby or sapphire)

Lights up past Segment 9 (three blue lights and bleeps) = diamond

Before the next test you must wait for the READY light. Repeatedly testing the same stone (especially a small stone) warms it until, eventually, a diamond will no longer read DIAMOND. Wait at least two minutes for the stone to cool down.


Turn the knob to light up the segments, set the starting-number like this:

METHOD 1: Look at the chart on the back of the diamond tester. The range of sizes and

the temperatures are not critical, use them merely as a guide.

METHOD 2: Memorise the following:

Unless you are working outside in the cold, start at No. 4 (or No.3 for large stones).

If you are working in the cold, look at the chart.

METHOD 3: You will soon get the 'feel' of how it works, in exactly the same way as getting the 'feel' of gears in a car or bicycle without having to think about it. The starting-segment is irrelevant, you will know if you have a diamond from the speed the lights move, and you will know that large / cold stones react faster and that small / warm stones react slower.


Knowing the principle upon which a diamond tester works is like learning which gear

numbers to use depending on the steepness of the hill and the speed of the car, it may

be of interest to the beginner but for the experienced it is totally irrelevant.

The tip of the tester warms up. Diamonds (and, to a lesser extent, rubies and sapphires)

are 'cold'. A crude test is to touch the stone against your lip to feel the 'coldness'. The

diamond tester measures this relative 'coldness', i.e. how quickly the heat from the tip is

'drawn' into the stone. The faster the heat is drawn into the stone, the faster and further

the lights move up. Of course, if the stone really is cold (e.g. outside in the winter) the

reaction will be quicker, or if the stone really is hot (e.g. from a hot shop window) the

reaction will be slower.


a small diamond won't have much 'coldness', it will 'draw' the heat slowly, the segments will light up slowly, you have to turn the sensitivity up to get it to read DIAMOND. A small non-diamond will be so 'warm' it won't draw the heat at all, the lights wont' movel. A large diamond will 'draw' the heat quickly, the lights will light up to DIAMOND in about one second. Some large non-diamond will draw some heat (especially rubies and sapphires), they will make the lights move up, but sluggishly, not at all like the reaction with a large diamond, but the chart will tell you to turn the sensitivity down slightly, just in case.



Supplied with battery inserted. To change battery slide battery cover open, insert one PP3 battery.


Gently remove the thin rubber protective cap from test tip.

Turn knob just enough for it to click. Wait for the READY light. This will take a few seconds.

Continue to turn the knob to set the sensitivity (see 'Setting the sensitivity').

Press the test tip firmly onto the top of the diamond at a right angle. Do not attempt to scratch the stone or you will break or bend the test tip and such damage is not covered by the guarantee.

If lights remain static you do not have a diamond;

If the row of lights light up rapidly (and there will be a bleep) you have a diamond.

Before the next test you must wait for the READY light

Both the stone and the diamond tester must be at room temperature. Repeatedly testing a stone warms it until a diamond will no longer read DIAMOND. The same applies to stones taken from a hot shop window. Conversely, very cold stones (especially large stones) will read DIAMOND e.g. if brought in from outside on a cold day. You MUST wait for the diamond tester and stone to return to room temperature or you WILL get wrong results.

Replace the protective cover after use.


Hold the diamond tester like a pen. Make sure that you are touching (with your hand) the metal plate on the back. Now if you miss the stone and touch the setting by accident there will be a piercing alarm. If you do not do this, touching the metal will register DIAMOND.


You can adjust the sensitivity to allow for cool temperatures or large stones.

METHOD 1: Set the LED lights to start on the number indicated on the chart on the back of the diamond tester.

METHOD 2: If it is not cold simply memorise the following:

SMALL STONES (under 5pts) Set LED lights to No.6

MOST STONES (6pts to 50pts) Set LED lights to No.4

LARGE STONES (60pts upwards) Set LED lights to No.2

Only if it is cold (below 10 degrees) need you look at the chart.

METHOD 3: You will soon learn that the starting number is irrelevant. If the LED lights light up rapidly and dramatically, you have a diamond; if they do not move at all you do not have a diamond; if they creep up slowly an painfully and bleep 'diamond' (especially with a large stone) then either you have set the sensitivity too high or you have a corundum (ruby or sapphire).

IMPORTANT: It is the SPEED the lights move that tells you if you have a diamond, the exact starting point is not important, you can therefore see if you have a diamond even if you start the lights at the wrong number. Small diamonds (especially if warm) will react very slowly, a large stone (any stone) will react dramatically if cold. Do not become obsessed about the lights 'tipping' into red and bleeping, it is the SPEED the lights move that tells you if you have a diamond. You will soon get the 'feel' of the machine - it's like getting the 'feel' of gears on a car or bicycle, you might know, academically, that you must change to a lower gear when going uphill (like starting on a lower number when testing a large diamond) but you don't have to refer to a chart.


The Battery: if the lights fade or don't light up at all, or if only some of the segments light up, or if the lights zip up-and-down, change the battery and use a good quality 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 and bargain stores last 1 to 2mns, similar bought in supermarkets can last several minutes. You must use a good quality alkaline battery such as Duracell or UCAR. They cost about £4.00 instead of £1.50 to £2.50 but they do last several months (providing you remember to switch the diamond tester off after each use). If you are still having problems with the diamond tester then feel free to send it back, but if we find the problem is the battery we will charge £4.50 for a new battery + £2.50 for return postage.

The Lights: There are three big holes, the bottom one is labelled READY and has a light in it, the middle one is label BATTERY and has light in it, the top one has no label at all and has no light - this hole is not used, it serves no purpose, it is not supposed to have a light in it. (The reason is that the casing is standard, it is used by many manufacturers for many different machines, and when it is used to make a diamond tester the extra hole isn't used - it's cheaper for manufacturers to use a standard casing and not use some of the 'holes' than to spend several thousand pounds getting a new casing made).

(This isn't really a problem with the DT-5 model because the labelling has been redesigned and is clearer)

Temperature: The stone and the tester MUST be the same temperature, ideally room temperature, but definitely the same temperature. Repeatedly testing the same stone warms it until, eventually, a diamond will no longer read diamond. If this happens wait 2 to 3 minutes for the stone to cool down. Similarly, testing a stone taken from under the lights of a hot window, or having been brought in from outside on a cold day, or even a small stone on a hot customer's finger - you will get false readings, you must wait for the stone to reach room temperature.

(In practice, the DT-5 model copes with all but the most extreme temperatures, but great care must be taken with other models)

Continuity: hold the diamond tester with your finger touching the metal plate, then if you miss the stone and touch the metal setting by accident you will hear a rapid-pip alarm (and the LED lights remain static) - however both hands must touch the metal to create a 'circuit' - i.e. do not wear rubber gloves, do not 'test' the machine on metal objects sheathed in plastic.

Sight: you must place the tip at a right angle in the centre of the stone, if you repeatedly hear the rapid-pip sound (rather than the bleep-bleep sound) this means you are missing the stone and touching the metal. You may need to find your reading glasses, you may need a magnifier, you may need to find someone with a steadier hand, you must place that tip firmly and at right angles in the middle of the stone.

Not covered by the guarantee: breaking the tip, cutting or pulling out the wire that connects the battery (though we do have spare connectors), dropping it, soaking it in tea or coffee or leaving it in the rain.

The tip: treat it with care, keep the protective cap on it when not in use (and put it back in its pouch), do not try to scratch the stone with the tip - it will bend if you do that! If you bend it, it is better to bend it back with jewellers pliers rather than throw it away, but if you then snap the tip off you will have to throw it away. Bending or breaking the tip is not covered by the guarantee. There no reason to ever bend or break the tip if you treat it carefully, these diamond tester do last many years...providing you don't bend or break the tip.


Moissanite is a synthetic stone, it is the closest man has yet come to imitating diamond, it is nearly as hard as diamond, and it does appear, at first glance, just like diamond and it registers DIAMOND on all diamond testers.

Some questions and answers:

Q. Will a DIAMOND TESTER distinguish diamond from Moissanite?

A. No. Diamond testers work on Thermal Conductivity and the thermal conductivity

of the stones is too similar to make this a reliable test

Q. Is there a tester that will distinguish Moissanite from diamond?

A. Yes, we sell electronic 'Moissanite testers' but these will only distinguish Moissanite from diamond, you will still need the DIAMOND TESTER to distinguish

diamond from dozens of other similar-looking stones.

Q, Is there one tester that will test for both diamonds and Moissanite?

A. Yes, but they are not as reliable as having two separate testers. The beauty of the

DT-5 diamond tester is that even if you set the sensitivity wrongly, it is still obvious

as to whether you have a diamond or not. The combined testers requires you to

follow the instructions very precisely, including cleaning the stone thoroughly with

alcohol, and also requires an extra-steady hand and perfect eyesight.


The stone must be absolutely clean, preferably by cleaning with a cotton wool bud soaked in alcohol. A layer of grease can give a false reading, showing diamonds to be Moissanite.

A very small number of stones were giving a false reading. It has been found that the solution is to carry out the test whilst shining a UV light (included) on the surface of the stone. The UV light does not have to be touching the stone, simply stand it on the table facing the stone and make sure it is switched on.

Hold the tester with your thumb over the test button and your finger touching the rear metal plate.

Squeeze your fingers so that you are pressing the test button.

The yellow 'testing' light should show. (No yellow 'testing' light? - change battery)

With your other hand hold the metal part of the jewellery (or stone holder, supplied)

Place the probe in the centre of the stone as far away as possible from the setting / metal stone-holder (if the probe is very close to metal, it can arc and give a false reading).


A MOISSANITE TESTER is designed to test stones that have already been tested with a DIAMOND TESTER and have already shown up as 'DIAMOND'.

Some customers return their MOISSANITE TESTER complaining that, "All sorts of things show up as Moissanite even though they can't possibly be Moissanite!". They are right. Parts of the human body, children's toys, lumps of rock from the garden - all sorts of things show up as Moissanite but none show up as 'DIAMOND' on a DIAMOND TESTER.


The combined DN1 diamond and Moissanite tester is not like our diamond-only tester. With the diamond tester you can be significantly 'wrong' with the settings and still get a clear result, with the DN1 you must follow the instructions very precisely. Providing you do this, the DN1 should prove a reliable and invaluable tester. But if you don't follow the instructions very precisely you will simply get wrong results.

Turn on with knob (this is merely an on/off knob, it does not adjust anything).

Wait until WAIT light goes off and READY light comes on.

Hold the DN1 so that your fingers of one hand are touching the metal plate and the other hand is touching the metal of the jewellery (or if the stone is not mounted, the metal of the stone-holder).

Place the test-tip firmly in the centre of the gem stone, and at right angles to the stone.

Simulant or non-diamond: no reaction

Diamond: buzzer + green light for about six seconds

Moissanite: within ½ second, tone will change + light changes from red to green


The DN1 tests for diamond (thermal conductivity) then ½ second later it tests for Moissanite (electrical resistance). These are two entirely different principles, so if you remove and replace the test tip from the stone within this one second, you will confuse the machine.

Touch the test tip firmly onto the stone and at a right angle, be careful that it doesn't slip. Do not press hard (as if trying to scratch the stone) - you could bend or snap the tip (not covered by the guarantee).

The stone must be absolutely clean, preferably by cleaning with a cotton wool bud soaked in alcohol. Greasy stones give a wrong results, cleaning by rubbing against your shirt is not good enough.

The tester is designed to test diamond-like gemstones. You may wish to experiment to see how many substances and household items have the same electrical resistance as Moissanite - a 'Moissanite' reading on these does not mean that the tester is broken since such substances / items cannot possibly be confused with diamond.

Unlike our diamond-only testers, the DN1 does not have a 'touched metal' safety feature, this means that if you miss the stone and touch the metal by accident you will get a 'Diamond' reading. Be quite certain that you can see the test-tip and the stone clearly even if this means finding your reading spectacles or using a magnifier.

If the yellow READY light fails to show or if it flashes, change the battery. The diamond test works on heat, use a good quality alkaline battery (the energy used to heat the tip will drain a cheap battery in minutes).


We sold a diamond tester to a man who went out to South Africa to buy 'bargain' 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. And 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.

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

Most importantly, 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. Well, perhaps if you go, in person, to the mines in the most war-torn parts of the world, traveling for days through barren land, braving road blocks and gunmen...

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 South 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 four stones to check the machine.

Tricksters have been know 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're 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 in everyday dealing where the amounts of money involved are relatively small, there is no need to become paranoid!