The best diamond testers, compared.
Top pick: NIUPIKA diamond tester Best value: QWORK LED diamond tester – Cheapest pen with a review file large enough to judge a costume screen: about £9.99, 3.9 from 124 reviews. Moissanite is not named. Updated October 2026.
A diamond tester is a handheld pen that heats a probe and reads thermal conductivity. In the UK you buy one to screen a ring from a private sale before you treat a colourless stone as diamond. You recognise a suitable pen by a steady ready lamp on a fresh cell, and by whether a valuable stone still goes to a jeweller or a laboratory. A thermal reading can often show that glass or cubic zirconia is not diamond. The same beep is also what synthetic moissanite produces. It does not separate a mined diamond from a laboratory-grown diamond. Lights for ruby, sapphire, jade or agate are a chart, not a species identification. The pen does not hallmark gold or silver. Hold the tip at a right angle on the table of the stone, with a bare finger on the metal plate. A cold stone can read as diamond, and no ready light means there is no test. How a thermal pen differs from a dual tester and from a laboratory is in the [diamond tester buying guide](/hub/measuring-equipment/diamond-tester/sub/diamond-tester-buying-guide).
A diamond tester is a handheld pen that heats a probe and reads thermal conductivity. In the UK you buy one to screen a ring from a private sale before you treat a colourless stone as diamond.
38 products checked · 56 hrs invested · +18,600 readers
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Personal buying guide
Question 1 of 2
What are you screening?
NIUPIKA HBM0009 is the most-reviewed pen in the set: £22.59, 4.0 from 305 reviews, with large and small stone switches and a 1 x 9V cell that is not included. The listing says it indicates genuine diamond or fake, and it does not print a ruby or jade chart, which is narrower than several £10 pens. UK texts still repeat three problems: no battery in the box, a unit that does not start, and the same reading on several materials. Some reviewers say it separated diamond from cubic zirconia or glass when it did start. A case is not confirmed in the package list. One UK review wanted a silver result and did not get one. The 4.0 line is not a reason to ignore the dead-unit comments. It can suit a costume check if you will return a pen that never shows ready. It does not admit the moissanite limit.
Pros
Cons
Best for: You want the most-reviewed thermal pen under £25 and will return one that never becomes ready.
Less suitable: You are ranking by stars alone, testing silver, or need the moissanite limit written on the page.
Specifications
Common questions
Does it come with a battery or a case?
The package list is 1 diamond tester. A 1 x 9V battery is not included. A case is not confirmed.
Can it test silver?
The bullets say it indicates genuine diamond or fake. One UK review wanted it for silver and did not get a useful result.
What do UK reviews repeat?
Several call it easy and good value, and some say it separated diamond from cubic zirconia or glass. Repeated complaints are no battery, a unit that does not start, and the same reading on several materials.
Divinge’s £17.59 kit pairs a thermal tester with two LED loupes, one described as 60x and one with 30x and 60x lenses, and the amazon.co.uk line is 3.9 from 138 reviews. The pack is listed at 21.5 x 12.8 x 4.1 cm and 270 g. Moissanite is not named, so a diamond beep still has the usual thermal limit. Stones in the bullets are diamond, sapphire, ruby, crystal, agate and jadeite, and the listing says to set the light to 5 on glass for other gems, which is a chart step rather than a species test. Both battery fields say No, while reviews disagree about whether a cell arrived fitted. Comments on the loupes split: one calls the 30/60x loupe decent, others say the image is reversed or the magnifier is useless. One review also says an item in the picture was missing. It is a looking kit plus a thermal pen, not a dual conductivity tester.
Pros
Cons
Best for: You want a thermal pen and a loupe in one parcel for costume pieces, and you will judge the magnifier yourself.
Less suitable: You need a moissanite limit in writing, or you are buying the kit only for the loupe picture.
Specifications
Common questions
What comes in the kit?
The listing says 1 jewellery tester and 2 loupes, one 60x with LED and one with 30x and 60x lenses. UK reviews also mention a mini microscope. One review says an item in the picture was missing.
Does a battery come with it?
Batteries-included and batteries-required both say No. One review says a battery is needed. Another says a cell was already fitted.
What do reviewers say about the loupes?
One calls the 30/60x loupe decent. Others say a magnifier is useless or shows the image reversed or upside down.
The QWORK WD40146 is a thermal LED pen at £9.99 that lights and beeps when the probe meets a stone, and the listing says the 9V cell is not in the box. Across 124 amazon.co.uk reviews it sits at 3.9, the largest sample under £11 in this set, so you can see both stones sorted from glass and units that never settle. The bullets name diamond, sapphire, crystal, agate and jadeite, and they do not name moissanite, so a diamond indication is still allowed to be synthetic moissanite. The page does print a usable hold: the probe at 90 degrees, a bare finger on the rear metal panel, no gloves, and do not test metal. The operating range is given as 5°C to 35°C with humidity under 80%. It is the best value here when you want a cheap costume screen and will buy a fresh alkaline 9V yourself. It is the wrong spend if the stone is worth more than the pen.
Pros
Cons
Best for: You want a low-cost thermal screen for costume jewellery you already suspect is glass or cubic zirconia, and you will fit a 9V cell.
Less suitable: The stone is valuable, or you need a listing that states a moissanite limit.
Specifications
Common questions
Does the listing include a battery?
The bullets say a 9V battery is not included. The batteries-included and batteries-required fields both say No.
Does the listing name ruby?
A listing prompt asks about rubies. The bullets name diamond, sapphire, crystal, agate and jadeite. Ruby is not in that list.
How does the listing say to hold the probe?
The probe needle at 90 degrees to the object, with the right index finger on the metal panel at the back. It says do not test metal and do not wear gloves.
Ignara’s GS-diamond selector is £10.59, with batteries required and no cell size printed, and a listed weight of 10 g that sits far below the other pens in this table. Stars are 3.9 from 11 reviews. One UK review calls the readings very inconsistent. Another says it seems fine as a budget tester and that it does not name the stone type, only a hardness or conductivity check. A third calls it a fire safety hazard in a care home. That last line is one review, not a recall, and it is still a reason not to leave the pen unattended if it heats. The title names moissanite without describing an electrical test. The included-components field says no. Treat the 10 g figure as a listing line this file does not verify on a scale.
Pros
Cons
Best for: You want to see how thin the evidence is before you pay £10 for an unnamed cell size.
Less suitable: You need a stated battery, a believable weight, or a pen without a heat complaint in the file.
Specifications
Common questions
Is a battery required?
Are-batteries-required says Yes. The included-components field says no. No cell size is printed.
What weight is printed?
10 g.
What do UK reviews say it does?
One says it seems fine as a budget tester and does not name the stone type; it tests hardness or conductivity. Another calls the readings very inconsistent. One calls it a fire safety hazard.
The Gain Express JEM-II at £55.99 is the top pick in this Amazon set when you want a small-stone thermal screen that also lists a UV lamp, a loose-stone platform and a vinyl pouch. The claim is stones down to 2 points, about 0.02 carat, and the star line is 3.8 from 179 reviews. A 9V cell is named and not included, and an AC adaptor is mentioned without being confirmed in the box. The seller says the tester is unresponsive to cubic zirconia and moissanite. No UK review in this file confirms that sentence on a named moissanite, and a matching JII-style note says that style of tester cannot distinguish synthetic moissanite. UV here is listed for a fluorescence look, not for a mined-versus-laboratory split. You are paying for the platform, the size claim and a larger review file than the £10 pens, not for a new physical principle. A valuable colourless stone still needs a jeweller or a laboratory.
Pros
Cons
Best for: You screen small loose stones you already suspect are glass or cubic zirconia, and you want a platform in the parcel.
Less suitable: You need a proven moissanite split, or the stone is valuable enough for a grading report.
Specifications
Common questions
How small a stone does the listing claim?
Stones as small as 2pt.
What is in the box?
A vinyl pouch, a testing platform for loose stones, and an instruction manual. The battery is not included. A 9V cell is named.
What does the listing say about moissanite?
It says the tester is unresponsive to imitation crystals and simulants such as cubic zirconia and moissanite. No UK review in the file confirms that on a named stone. UV is listed for fluorescent reaction testing.
Vakler DTP01 is a £9.99 thermal pen, 16 x 4 x 2.2 cm and 100 g, that puts moissanite in the title beside diamond, jade, ruby, sapphire, agate and crystal. The amazon.co.uk line is 3.8 from 25 reviews, a small file next to the QWORK pens at the same price. The word moissanite in the title is not an electrical conductivity test, so a diamond beep can still be that silicon carbide. One UK review says smoke came out of the unit, which is a reason to stop using that pen, not a published recall of the model. Another review says the control changes sensitivity rather than volume, and that a rechargeable 9V upset the setting. The cell is 1 x 9V and is not included. Buy it only after you have read those texts. Skip it if you wanted a confirmed moissanite split.
Pros
Cons
Best for: You have read the smoke review and still want this £9.99 listing for a costume screen.
Less suitable: You want a confirmed moissanite split, or you will not return a unit that smokes.
Specifications
Common questions
Which stones does the listing name?
Diamond, moissanite, jade, ruby, sapphire, agate, crystal and other precious stones.
What battery does it need?
1 x 9V, not included. One UK review says the cell is hard to remove and that a rechargeable 9V upset the sensitivity setting.
What went wrong in the UK reviews?
One review says smoke came out. Another says the control changes sensitivity rather than volume, and that the pen beeped without touching a stone at a higher setting.
VivaStride’s pen is £10.99, 13.5 x 4 x 2.3 cm and 140 g, with 3.8 from only three reviews. The bullets name diamond, sapphire, ruby, crystal, agate and jade. The only UK review says it beeps and shows red for diamond and for moissanite, and that it sorts those from cubic zirconia, not from sapphire, ruby or agate. That single text is more useful than the gem list, and it is still one person’s trial. The highlight says the pen comes with a battery, while the batteries-required field says Yes. A listing prompt asks whether the probe can be replaced; this file does not say the tip can be swapped. If you already own it, trust the review’s limit over the bullet list. Do not buy it as a ruby or jade identifier.
Pros
Cons
Best for: You already own it and will treat a red beep as diamond or moissanite, not as a named coloured gem.
Less suitable: You are choosing a first pen and want more than one UK review.
Specifications
Common questions
Which stones does the listing claim?
The bullets name diamond, sapphire, ruby, crystal, agate and jade. The only UK review says it beeps and shows red for diamond and moissanite, sorts those from cubic zirconia, and does not cover sapphire, ruby or agate.
Is a battery included?
The highlight says with battery. The batteries-required field says Yes.
Can the probe be replaced?
A listing prompt asks. The listing does not say the probe is replaceable.
The second QWORK, model WD43178, is another £9.99 thermal LED pen that claims nine or more lights and a beep for diamond, and seven lights for emerald, crystal and agate. The star line is 3.6 from 84 reviews, weaker than the other QWORK at the same price. Setup text says insert a battery, while both battery fields say No, and one UK review says it was not obvious that no cell was included. Calibration is described as setting the pen on glass. Named stones are diamond, sapphire, ruby, agate and jade, which is a light chart rather than a laboratory identification. Weight is listed at 85 g and the working band is 5–35°C with humidity under 80%. Choose it only if this exact listing is already in your basket and you accept the battery clash.
Pros
Cons
Best for: You already have this ASIN in the basket and only need a glass-or-diamond screen after you add a cell.
Less suitable: You want the larger review file at £9.99, or a pen whose battery line agrees with itself.
Specifications
Common questions
What temperature does the listing give?
5–35°C, and relative humidity under 80%.
Is a battery in the box?
The setup steps say insert a battery. The batteries-included and batteries-required fields both say No. One UK review says it was not clear that no battery is included.
Which stones does the listing name?
Diamond, sapphire, ruby, agate and jade. It also claims 9 or more indicator lights and a beep for diamonds, and 7 lights for emerald, crystal and agate.
The Gain Express DMT-3 at £104.90 is the only listing in this set that says it is for natural diamonds and not suitable for laboratory-grown diamonds, and that claims a split of diamonds and moissanite from fake stones. Stones from 2 points are claimed. Power is AAA alkaline, NiMH or an AC adaptor, none included, and the operating band printed is 18°C to 30°C. The bullet size is 173 x 42 x 22 mm and 65 g without batteries, while another line says 22 x 5 x 12 cm. Stars are 3.4 from 63 reviews. Of five short UK lines, three say it does not work, including a beep on steel. Reviews from other marketplaces in the notes say moissanite was read as diamond or flipped between moissanite and diamond on a retest. The laboratory-grown sentence is a limitation, not a detection feature. At this price a specialist dual tester sits further up the ladder, outside this table.
Pros
Cons
Best for: You want the only Amazon listing in this set that writes the laboratory-grown limit in the bullets, and you accept the review complaints.
Less suitable: You think that sentence means the pen finds laboratory-grown diamonds, or you need a dual tester with a cleaner review file.
Specifications
Common questions
What does the listing say about moissanite and lab-grown diamonds?
It says the tester distinguishes diamonds and moissanite from fake stones, and that it is for natural diamonds and not suitable for lab-grown diamonds.
How small a stone, and how is it powered?
Stones as small as 2pt. Power is AAA alkaline, NiMH, or an AC adaptor, none included. The bullet size is 173 x 42 x 22 mm and 65 g without batteries. Another line says 22 x 5 x 12 cm.
What do the reviews say?
The five UK lines are short: two are positive and three say it does not work, including a beep on steel. Other reviews in the notes say moissanite was read as diamond, or the same stone flipped between moissanite and diamond.
Jiyeo’s £9.99 pen names diamond and moissanite on an LED-and-beep body listed at 110 g. The entire amazon.co.uk file is 2.5 from three reviews: one stopped turning on after a few days, one says the item is not working, and one says it separated diamond from non-diamond stones so far and was easy to use. Power is described as battery powered, while the batteries-required field says No, and no cell size is printed. The materials line says alloy steel and plastic. Three reviews cannot give you a failure rate, yet two of the three already describe a dead unit. The moissanite word in the title does not add an electrical test. Leave this ASIN unless a later, larger UK sample appears on the same page.
Pros
Cons
Best for: You will only keep it if it switches on the day it arrives, and you already accept a three-review file.
Less suitable: You need any working costume screen this week.
Specifications
Common questions
Does the listing say a battery is required?
Power source is battery powered. Are-batteries-required says No. No cell size is printed.
What do the three UK reviews say?
One says it stopped turning on after a few days. One says the item is not working. One says it distinguished diamond from non-diamond stones so far and was easy to use.
What is the pen made of, on the listing?
Alloy steel and plastic. Weight is 110 g. Size is not printed.
The RIZGHWOY kit is £16.89 and claims a thermal check, about 20 seconds to ready, a platform and an illuminated loupe, with a 9V cell that is not included. Stars are 2.4 from six reviews, the weakest line in the set. UK reviews say it does not distinguish silver from base metal and does not work as a precious-metal tester, which matters because a listing prompt invites that job. The bullets describe a gem check and a metal alert, not an assay mark. Included-components says No while the bullets name a platform and a loupe, and one review says the fitted case shown on the page was not supplied. One reviewer still describes a small diamond separated from a replica, and a change in sound when the tip hits the claw. At this price you are paying more than the £9.99 pens for a confused box list.
Pros
Cons
Best for: You are checking whether a kit page matches the parcel before you pay £16.89.
Less suitable: You want to test silver, hallmarks, or documents, or you want a coherent box list.
Specifications
Common questions
Does it test silver?
A listing prompt asks about silver. UK reviews say it does not distinguish silver from base metal and does not work as a precious-metal tester. The bullets describe a thermal gemstone check and a metal alert.
What is in the box?
The bullets name a testing platform, an illuminated loupe, LED lamps and a buzzer. The included-components field says No. A 9V battery is not included. One review says the fitted case on the page was not supplied.
How does a reading work, according to the reviews?
One reviewer describes a thermal go or no-go test. The listing says about 20 seconds to ready and calibration for stone size. That review says a small real diamond was separated from a replica, and that touching the metal setting changes the sound.
Eleven diamond-tester listings on amazon.co.uk were scored on 20 October 2026 from listing facts and review notes. This is not a laboratory test. A beep is not proof of a mined diamond, and stars are not an accuracy figure. Each score starts at the star rating times 16. Review depth, a stated 2-point size, a platform or large/small switch, a UV lamp, and a written laboratory-grown limit can add points. Dead units, an unconfirmed moissanite claim, a single smoke or fire-hazard review, a silver or document job the pen does not do, loupe complaints, and clashing battery fields come off. The highest score is the top pick. The best value is the cheapest pen with a review file large enough to judge a costume screen. The four weights below group that note and sum to 100. The score on each row is the point total, not a percentage.
Amazon star line
45%The base of the score is the amazon.co.uk star average times 16. A 4.0 line starts higher than a 2.4 line. Stars are not an accuracy figure for the stone.
Review file and faults
25%Review depth adds 6 points at 100 or more reviews, 3 at 50 or more, and subtracts 6 under 10 reviews. Repeated dead units or the same reading on several materials subtract 8. A single smoke or fire-hazard review subtracts 4. Loupe complaints subtract 3.
Stated job
20%A printed 2-point size adds 4. A loose-stone platform or a large/small switch adds 3. A UV lamp listed beside the thermal probe adds 3. A listing that states it is not for laboratory-grown diamonds adds 6. An unconfirmed or contradicted moissanite claim subtracts 6.
Listing consistency
10%Battery fields that contradict the setup text subtract 2. A page that invites a silver or document test the reviews say the pen does not do subtracts 4. A diamond result is still not a hallmark.
Start with the method, then what a diamond beep is allowed to mean, then the battery and the probe, then stone size and temperature, then price against the job. The notes below follow that order. None of the scores is a laboratory test.
A diamond tester is a handheld pen that heats a probe and reads thermal conductivity. Shoppers on amazon.co.uk use one to screen a ring from a private sale or a car-boot find. A working thermal pen can often show that glass or cubic zirconia is not diamond. A diamond beep is also what synthetic moissanite produces. GIA’s 1997 study found that thermal probes treat that silicon carbide as diamond.
A thermal diamond result may be diamond, synthetic moissanite, a cold stone, or a sensitivity error. It does not separate mined diamond from laboratory-grown diamond.
Lights for sapphire, ruby, agate or jadeite are a chart on the pen. They are not a species identification.
A diamond tester does not hallmark metal. Gold, silver, platinum and palladium over the legal weight thresholds need an Assay Office mark. A metal beep is not a result.
Thermal conductivity rules out many simulants, including glass and cubic zirconia, and it cannot clear synthetic moissanite. A dual thermal-plus-electrical tester is the next rung, and laboratory-grown screening is another instrument. GIA still says a definitive answer is a laboratory. Most pens here need a 9V cell that is not in the box. No steady ready lamp means no test. The tip meets the table of the stone at 90 degrees, with a bare finger on the metal plate.
Under about £25 in this set you are buying a costume screen. UK reviews of several of those pens repeat units that do not start, so the cheapest shelf price is not the best value unless the review file is large enough to judge. The best value here is the £9.99 pen with 3.9 from 124 reviews. About £56 to £105 buys a platform, a 2-point claim, or stronger wording, not a new physical principle. The £104.90 listing writes that it is not for laboratory-grown diamonds. That is a limit.
Three typical situations, each with a recommendation from this comparison.
You already suspect the stone is glass or cubic zirconia and you will fit a fresh 9V cell. The QWORK LED pen is about £9.99, with 3.9 from 124 reviews. Moissanite is not named, so a diamond beep can still be that simulant.
You want a platform in the parcel and a listing that claims stones down to 2 points. The Gain Express JEM-II is about £55.99, with 3.8 from 179 reviews, a UV lamp and a vinyl pouch. The seller’s moissanite line is not confirmed by a UK review in this file.
You want the only listing in this set that says it is not for laboratory-grown diamonds. The Gain Express DMT-3 is about £104.90, with 3.4 from 63 reviews. That sentence is a limitation, not a detection feature. A valuable stone still needs a grading report.
In this set of eleven amazon.co.uk listings, the top pick is the Gain Express JEM-II at about £55.99, with 3.8 from 179 reviews, a UV lamp, a loose-stone platform, a vinyl pouch and a 2-point claim. The seller’s moissanite line is not confirmed by a UK review in this file. The best value is the QWORK LED pen at about £9.99, with 3.9 from 124 reviews, the largest sample under £11. Moissanite is not named on that listing. A beep is not proof of a mined diamond, and none of these scores is a lab test.
QWORK LED diamond tester
Cheapest pen with a review file large enough to judge a costume screen: about £9.99, 3.9 from 124 reviews. Moissanite is not named.
Editorial
Peter F. is the editor for DIY & tools at productscompared.co.uk. Comparisons in this category are checked against amazon.co.uk listings, review evidence and public UK sources.
Guides on what to look for, how to use a product, and common mistakes.
Public sources listed below, last checked on 20 October 2026. Shelf prices and star lines are the amazon.co.uk figures captured with these eleven listings. The price bands are this set, not a market average. Quicktest figures are shop estimates.
This page contains affiliate links to amazon.co.uk. If you buy through them, we may earn a commission at no extra cost to you. Prices, stars and review counts are from the UK listings checked on 20 October 2026. We did not lab-test these diamond testers.
