Best diamond tester
Updated: 22 October 2026
You want the best diamond tester in a basket of eleven amazon.co.uk pens, and a single winner is the wrong shape for that question. A £9.99 pen that switches on can screen costume jewellery you already suspect is glass. The same pen is a poor buy when the stone would hurt to lose. Prices in this set, captured on 20 October 2026, run from four listings at £9.99 up to the Gain Express DMT-3 at £104.90. The [diamond tester comparison](/hub/measuring-equipment/diamond-tester) holds the full table. This page gives each band a job, including the job of buying nothing. Shop instruments that combine heat and electricity sit outside the set. Those prices live on the [diamond tester buying guide](/hub/measuring-equipment/diamond-tester/sub/diamond-tester-buying-guide). A qualifying purchase can earn a commission at no extra cost to you.
You want the best diamond tester in a basket of eleven amazon.co.uk pens, and a single winner is the wrong shape for that question. A £9.99 pen that switches on can screen costume jewellery you already suspect is glass.
You want the best diamond tester in a basket of eleven amazon.co.uk pens, and a single winner is the wrong shape for that question. A £9.99 pen that switches on can screen costume jewellery you already suspect is glass. The same pen is a poor buy when the stone would hurt to lose. Prices in this set, captured on 20 October 2026, run from four listings at £9.99 up to the Gain Express DMT-3 at £104.90. The diamond tester comparison holds the full table. This page gives each band a job, including the job of buying nothing. Shop instruments that combine heat and electricity sit outside the set. Those prices live on the diamond tester buying guide. A qualifying purchase can earn a commission at no extra cost to you.
Our top picks
| Badge | Product | Type | Moissanite | Extras | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  QWORK QWORK LED diamond tester | Thermal LED pen | Not named | User manual stated | 6.8 | about £9.99 | View offer |
 Gain Express Gain Express JEM-II diamond tester | UV and thermal pen | Seller claim: unresponsive to moissanite | Vinyl pouch, loose-stone platform, manual | 7.2 | about £55.99 | View offer | |
| Alternative |  Gain Express Gain Express DMT-3 diamond tester | Pen with a claimed moissanite split | Listing says it distinguishes moissanite from fakes | Case not named | 6.0 | about £104.90 | View offer |
On this page
Under £25, a costume screen
The entry cluster is four pens at £9.99 (two QWORK models, Vakler and Jiyeo), Ignara at £10.59, VivaStride at £10.99, the RIZGHWOY kit at £16.89, the Divinge loupe kit at £17.59 and the NIUPIKA at £22.59. Quicktest treats a thermal pen under £50 as a costume-jewellery screen. You are paying for a heated tip that might show glass or cubic zirconia is not diamond, if the pen starts.
NIUPIKA is the highest line, 4.0 from 305 reviews, and the notes also repeat units that do not start, a missing 9V cell, and the same reading on several materials. Some reviewers say it separated diamond from cubic zirconia or glass when it became ready. Moissanite is not named. A 4.0 does not erase the dead-unit texts.
The rest of the band is thinner. Jiyeo is 2.5 from three reviews, and two say it does not work. VivaStride is 3.8 from three reviews, and the only UK text says it beeps for diamond and moissanite and sorts those from cubic zirconia. RIZGHWOY is 2.4 from six reviews and is not a silver tester. Vakler is 3.8 from 25 reviews, with one smoke review. Ignara is 3.9 from 11 reviews, with one care-home fire comment and readings called inconsistent. Those safety lines are single reviews, not a recall. Divinge at £17.59 and 3.9 from 138 reviews adds two loupes that reviewers disagree about. A moissanite word in a title is not an electrical test.
If you will return a pen that never shows ready, compare which page has enough UK reviews to show that failure. The QWORK LED pen at £9.99 has 3.9 from 124 reviews and prints the 90-degree hold. It still does not name moissanite.
£55.99, accessories and a 2-point claim
The Gain Express JEM-II is about £55.99, with 3.8 from 179 reviews. The listing adds an ultraviolet lamp, a vinyl pouch, a platform for loose stones and a claim down to 2 points, about 0.02 carat. A 9V cell is named and not included. An AC adaptor is mentioned and is not confirmed in the box. For melee that £10-pen reviews say they miss, the printed size and the platform are the reason the price moves, if you accept that both are seller claims.
The seller says the tester is unresponsive to cubic zirconia and moissanite. No UK review in these notes confirms that on a named moissanite, and a note on this JII style says it cannot distinguish synthetic moissanite. The ultraviolet lamp is a fluorescence look, not the electrical moissanite step, and it does not tell mined diamond from laboratory-grown diamond. Whether £55.99 is worth it depends on whether you test loose stones around 2 points and will still not trust a beep on anything valuable.
£104.90, a listing that names a limit
The Gain Express DMT-3 is about £104.90, with 3.4 from 63 reviews. It claims stones from 2 points and a split of diamonds and moissanite from fake stones, and it says it is for natural diamonds and not suitable for laboratory-grown diamonds. Power is AAA alkaline, NiMH or an AC adaptor, none included. The operating band is 18°C to 30°C. The bullet size is 173 x 42 x 22 mm and 65 g without batteries. Another line says 22 x 5 x 12 cm, so quote both if you check the parcel.
Five UK reviews are one line each. Two are positive. Three say it does not work, including a beep on steel. Notes from outside the UK review text say moissanite was read as diamond, or flipped between moissanite and diamond. The laboratory-grown sentence means this instrument is the wrong tool for that question. At £104.90 you are still short of a combined tester, which Quicktest prices from about £199.50.
When the best buy is no pen
Do not buy any pen in this set when the stone is worth real money, when you need a written identification, or when you are trying to prove the diamond was mined. IGI's handheld-tester article is plain that a thermal beep is not proof of a natural diamond. A diamond tester does not hallmark metal. GOV.UK's hallmarking summary is about the Assay Office, and RIZGHWOY's reviews are the worked example: people arrived expecting a silver or document check and did not get one.
A lamp for jade, ruby or sapphire on a £10 pen is not an identification you can repeat to a buyer. If two known stones already give nonsense, another pen from the same band usually repeats the fault. Why a diamond beep can still be moissanite is on the moissanite page. For a valuable colourless stone, the best diamond tester in this basket is the one you do not order.
Common questions
Which pen in this set is the best buy under £25?
The best buy under £25 is a thermal pen that switches on, for costume jewellery you already doubt, and that you can return if the ready lamp never appears. NIUPIKA has the most reviews and also a thick file of dead units. The QWORK LED pen at £9.99 has 124 reviews and a printed hold. Jiyeo, VivaStride and RIZGHWOY have only three to six reviews, which is too small to rank. None of them clears moissanite.
Does a 4.0 from 305 make the NIUPIKA the one to buy?
No. The 4.0 from 305 is the strongest star line in the set, and the same notes repeat pens that do not start, a 9V cell that is not in the box, and one reading across several materials. Some owners did separate diamond from glass or cubic zirconia. Stars without that split in the text would hide the failure. Buy it only if a return is easy when it stays dark.
Is the £55.99 JEM-II worth it over a £10 pen?
It is worth the gap if you want the printed 2-point claim, the loose-stone platform, the pouch and a review file of 179, and you understand the moissanite sentence is unconfirmed. It is not worth it if you hoped ultraviolet light would replace an electrical moissanite test. A £10 pen that starts does the same thermal job on a large costume stone. Neither replaces a jeweller when the price of the ring matters.
Does the £104.90 DMT-3 solve moissanite and lab-grown diamonds?
It does not. The listing claims a moissanite split and says the pen is not for laboratory-grown diamonds. UK lines include units that do not work. Notes from outside the UK reviews say moissanite still read as diamond. The laboratory-grown line is a limit, not a detector. A combined tester from a specialist shop is a different method, and even that method has misses.
When should I buy nothing from this set?
Buy nothing when you need a mined-versus-laboratory answer, a written report, a hallmark, or a named coloured gem. Buy nothing if you will treat any beep as proof you can pass to a buyer. A costume screen of a stone you already think is glass is the case that still fits a pen, and only after it shows a ready lamp.
Why do so many reviews say the pen is dead?
Most of these listings need a 9V cell that is not in the box, and several battery fields contradict the setup text. A flat cell, a rechargeable cell that upsets the setting, or a unit that never leaves standby all show up as a pen that does not work. Change to a fresh alkaline cell before you blame the stone. If it still will not show ready, that review pattern says return it rather than probe harder.
Can I use any of these on gold or silver?
No. The probe is a thermal check on a stone, and a continuous reaction on the claw is a metal alert, not an assay. Hallmarking of gold, silver, platinum and palladium is an Assay Office job once the piece is over the legal weight. A pen that beeps on a mount has touched the setting. It has not tested the metal.
Related pages
Sources
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.
- IGI: Diamond detection and handheld testers (retrieved 20 October 2026)
- GIA: Synthetic moissanite (Winter 1997) (retrieved 20 October 2026)
- GIA FAQ: identify laboratory-grown diamonds (retrieved 20 October 2026)
- GIA: Laboratory-grown diamonds update (Summer 2024) (retrieved 20 October 2026)
- Quicktest: guide to buying diamond testers (retrieved 20 October 2026)
- Quicktest: diamond tester troubleshooting (retrieved 20 October 2026)
- GOV.UK: hallmarking is the law (retrieved 20 October 2026)
- Consumer Rights Act 2015, section 9 (retrieved 20 October 2026)
- amazon.co.uk listings for the eleven ASINs (retrieved 20 October 2026)