How to use a diamond tester
Updated: 22 October 2026
You have a thermal pen on the table and a ring from a car-boot stall or a private sale. The lamp is dead, or the pen buzzes as soon as the tip meets the setting. How to use a diamond tester, for the pens in this [diamond tester comparison](/hub/measuring-equipment/diamond-tester), is the sequence that has to be true before one reading counts. Peter F. took the steps from specialist handbooks and the amazon.co.uk listings checked in October 2026, with no laboratory trial of the pens. A qualifying purchase can earn a commission at no extra cost to you. A steady ready lamp, a bare finger on the plate and the tip on the table of the stone are the minimum. Until those three are true, you are still setting the pen up.
You have a thermal pen on the table and a ring from a car-boot stall or a private sale. The lamp is dead, or the pen buzzes as soon as the tip meets the setting.
You have a thermal pen on the table and a ring from a car-boot stall or a private sale. The lamp is dead, or the pen buzzes as soon as the tip meets the setting. How to use a diamond tester, for the pens in this diamond tester comparison, is the sequence that has to be true before one reading counts. Peter F. took the steps from specialist handbooks and the amazon.co.uk listings checked in October 2026, with no laboratory trial of the pens. A qualifying purchase can earn a commission at no extra cost to you. A steady ready lamp, a bare finger on the plate and the tip on the table of the stone are the minimum. Until those three are true, you are still setting the pen up.
Our top picks
| Badge | Product | Power | Type | Extras | Note | Price | Offer |
|---|---|---|---|---|---|---|---|
| Best overall |  NIUPIKA NIUPIKA diamond tester | 1 x 9V, not included | Thermal pen | 1 tester. Case not confirmed. | 6.4 | about £22.59 | View offer |
| Best value |  QWORK QWORK LED diamond tester | 9V, not included | Thermal LED pen | User manual stated | 6.8 | about £9.99 | View offer |
 Gain Express Gain Express JEM-II diamond tester | Alkaline; 9V named; not included. AC adaptor option stated. | UV and thermal pen | Vinyl pouch, loose-stone platform, manual | 7.2 | about £55.99 | View offer |
On this page
Battery and the ready lamp
Most pens in this set need a fresh alkaline 9V, and many listings ship the body without one. Quicktest treats a flashing on-light, fading LEDs or a continuous buzz as a battery fault, and some high-power cells fade after a few tests. Fit an alkaline 9V and wait. On a Presidium Diamond Mate the ready light blinks through warm-up, and you test only after the blink stops. A lamp that will not settle is a reason to change the cell before the ring is part of the story.
The Vakler listing calls its control sensitivity, and a review said a rechargeable 9V threw the setting off. If a fresh alkaline cell still will not hold a ready state, stop. One Vakler review described smoke, so remove the cell and leave that unit alone. Take the 9V out for storage so it cannot leak. Which pen to buy is on the best diamond tester page.
Hands, angle and the metal alert
These pens close the circuit through your skin. The Presidium handbook, the RS Mizar Diamond Detective manual and the QWORK and Divinge sheets all want a bare finger on the metal plate. Cold hands from a January market make a poor contact, so warm them first. The tip belongs on the table of the stone at 90 degrees, and it should be free to retract. A slanted tip skids into the claw, and from then on you are reading the mount.
The Diamond Detective manual treats a continuous beep as the tip on a hand or on metal. A diamond indication on that instrument is a flashing lamp and a broken beep. Quicktest expects a result in one or two seconds, then you lift off. Holding on longer bends the probe and heats a small stone. UK reviews of the RIZGHWOY say the claw spoils the reading. Use a small/large switch only when your sheet prints one, as on the NIUPIKA. A clean diamond beep can still be moissanite, which is the moissanite page.
Temperature and a second try
The stone has to be indoors. Presidium's Diamond Mate handbook sets the band at 18–27°C, and the DMT-3 listing prints 18–30°C. Quicktest says a cold stone, including one just in from a UK winter market, can make a cheap thermal pen call a non-diamond a diamond. A stone you probed a minute ago is warm, and a hot stone can fail to read as diamond. On a small stone, wait before you repeat.
With the unit off, draw the tip across plain paper, the Presidium cleaning step, without bending the probe. The VivaStride listing does not say the tip can be replaced. Wipe jewellery cleaner off the table and let it dry before one short contact. If the stone is worth more than the pen and the lamp still will not settle, stop. The laboratory-grown diamonds page is the next question once the routine itself is no longer the doubt.
What the cheap instructions add
Divinge tells you to set other gems on glass until the display sits on light 5, and QWORK sits in that family of sheets. That glass step is a sensitivity set. A Presidium Diamond Mate uses a ready lamp that stops blinking, and Presidium says the owner should not self-calibrate that model. If your sheet really says light 5 on glass, use it for sensitivity, then still check the finger, the angle and the metal. Lamp counts for ruby, jade or sapphire do not name those species.
Battery fields clash on the same Amazon page. If the pen has a door, it needs a fresh alkaline cell even when a field says batteries are not required. The RIZGHWOY sheet mentions about 20 seconds to ready. That is warm-up, not a reason to hold the tip down once the pen is ready. JEM-II and DMT-3 listings claim a reach to 2 points. In a tight four-claw head the tip hits metal first. If the probe cannot sit on the table alone, that size claim has not produced a test.
Common questions
Why will the ready light not stay on?
A flashing or fading lamp is a cell fault before it is a verdict on the stone. Quicktest points to a weak battery, including high-power cells that die after a few tests, and these pens want a fresh alkaline 9V. On a Presidium Diamond Mate you wait until the warm-up blink stops. If a new cell still fails, or the pen has smoked as one Vakler review described, take the battery out and stop.
What does a constant buzz mean?
On the Diamond Detective manual a constant buzz means the tip is on a hand or on metal. Diamond on that unit is a flashing lamp and a broken beep, so lift off and check the claw. Quicktest also reads a continuous buzz, when the lamps are fading, as a dying cell. Fit a fresh alkaline 9V and try one short contact on the table. A buzz before the tip reaches the stone has not tested the gem.
Does the tip have to be straight down on the stone?
Yes. The tip should meet the table at 90 degrees and stay free to retract, which is how Presidium, QWORK and Divinge describe it. A slant walks into the claw and raises the metal alert. Harder pressure bends the probe. On a 2-point stone, the size the JEM-II and DMT-3 claim, the claw often leaves no bare table. If the probe cannot sit on the stone alone, stop.
Can I test a ring that just came in from outside?
Let it reach room temperature first. Presidium's band is 18–27°C and the DMT-3 listing says 18–30°C. Quicktest warns that a cold stone from a winter market can read as diamond when it is not, and that a stone you have just probed is too warm to read as diamond. One test, after the ring has settled indoors, is the reading. A second probe straight away only heats the stone.
How do I clean the tip without bending it?
Switch the pen off and draw the tip across plain paper, the Presidium cleaning step. Do not bend the probe to chase a speck. Quicktest says the tip must retract, and a bent tip spoils the next angle. A dirty tip, or jewellery cleaner left on the stone, makes the pen less sensitive, so wipe the table and let it dry. The VivaStride listing does not offer a replacement probe. If the tip is already bent, retire the pen.
Why do two tests of the same small stone disagree?
The stone was cold, still warm from the last probe, or the tip hit metal on one pass. Quicktest says to let a small stone cool between tests for that reason. Clean the tip on paper with the pen off, wait, then make one short contact at 90 degrees with a finger on the plate. If two careful tests still disagree, stop. Further probes only put more heat into the stone.
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)