A diamond tester works by touching a heated probe to the stone and measuring how fast heat moves through it, since diamond conducts heat better than almost any simulant.
A cheap moissanite ring passes the pen-shaped tester at the pawn shop, and the buyer walks out happy.
The short version: two different physical properties get tested, and only one of them separates diamond from moissanite. Here is how each one works, what the manuals actually tell you to do, and where handheld testers fall short.
The Two Physical Properties Behind Every Reading
Diamond conducts heat extremely well, and thermal testers exploit that single trait. Electrical conductivity is the second property, used only in “combo” models to catch the stones heat alone cannot separate.
On the thermal side, the probe tip is heated to a set temperature and then pressed against the stone. Because diamond carries heat away from the tip far faster than glass, cubic zirconia, or most other lookalikes, that rapid heat drain trips the response — typically a lit LED, a beep, or both. The stone itself is not damaged; the tester is reading a rate, not zapping the gem.
The catch is moissanite. Moissanite also conducts heat well enough to fool a thermal-only tester into reading it as diamond, which is why combo testers add a second channel. Diamond is an electrical insulator, while moissanite conducts a small electrical signal. A combo unit reads both properties and separates the two where heat alone cannot.
What The Testers Actually Measure
| Tester Type | What It Reads | Separates Moissanite? |
|---|---|---|
| Thermal-only pen tester | Heat transfer rate through the stone | No — moissanite often reads as diamond |
| Combo (thermal + electrical) | Heat transfer plus electrical conductivity | Yes — the electrical channel catches moissanite |
| Multi-tester with readout | Heat, electrical signal, and a numeric scale | Yes, with a display instead of a single light |
| Reflectivity / refractive tester | How light bends and reflects inside the stone | Sometimes — depends on the model and stone |
| Lab-grown diamond | Reads as diamond on any of the above | N/A — these testers cannot tell origin |
| Cubic zirconia, glass | Poor heat transfer, low electrical response | Reads as simulant on all types |
| Dirty or oily stone | Unreliable reading regardless of tester type | No — clean it before testing anything |
If you are weighing which model to buy, a tested comparison of handheld units is worth reading before you spend: our roundup of the best diamond testers walks through what each type actually does in practice.
How To Use a Handheld Tester Correctly
Follow the manual for your specific unit, since warm-up times and battery specs vary by model, but the sequence below reflects standard published guidance.
- Install a fresh battery. One manual specifies a 6F22 9V cell, so match whatever your unit calls for.
- Turn the unit on and let the probe warm up.
- Set the mode switch. Some models have a Diamond position that must be selected before testing.
- Clean and dry the stone. Use a gem cloth and make sure the surface is free of oil, lotion, or moisture.
- Hold the jewelry or loose stone securely, keeping body contact with the conductive plate if your model has one.
- Press the probe perpendicular and squarely onto a flat facet — never on an edge or point.
- Read the result.
The Diamond Detective owner’s manual is a useful reference for the exact sequence on that model — the Diamond Detective instruction manual covers warm-up, cleaning, and reading the light and beep signals.
Where These Testers Get It Wrong
Most bad readings trace back to handling, not the device. Testing a dirty, oily, or wet stone is the most common cause of a false result, and touching an edge or point instead of a flat facet both skews the reading and risks damaging the probe tip.
The bigger limitation is scope. A basic handheld tester tells you diamond versus non-diamond and related simulants — it generally cannot tell you whether a diamond is natural or lab-grown. Both read the same way, because the thermal and electrical properties are identical.
Moissanite remains the headline caveat. On a thermal-only unit, a moissanite stone can produce a full diamond response. If you are testing anything that could be moissanite — and moissanite is extremely common in modern jewelry — a combo tester is the practical answer, not a better thermal pen.
For stones where origin or exact identity matters on a high-value piece, no handheld unit replaces a gemologist using a full lab setup. A tester is a fast screening tool, and it is excellent at that job.
FAQs
Can a diamond tester damage my stone?
No. The probe heats a small tip and measures how quickly heat leaves it — the stone is never zapped or altered. The real risk is to the tester itself: pressing the tip against an edge or point can chip the probe, which is why the manuals say to test on a flat facet.
Why does my tester beep on moissanite?
Moissanite conducts heat well enough to trigger thermal-only testers, so the unit reads it as diamond. The electrical channel in a combo tester exists specifically to catch this, since diamond is an electrical insulator and moissanite is not. A thermal-only pen cannot resolve the difference.
Can these testers tell lab-grown from natural diamond?
No. Lab-grown and natural diamonds share the same thermal and electrical properties, so both light up the tester identically. Handheld testers only indicate diamond versus non-diamond and related simulants. Confirming origin requires a gemologist and lab equipment.
Does a dirty stone really change the reading?
Skin oil, lotion, dust, and moisture all interfere with how the probe transfers heat. Cleaning with a gem cloth and letting the stone dry before testing fixes most inconsistent readings, and it costs nothing to do.
References & Sources
- Our Weigh. “Diamond Detective Owner’s Manual.” Covers warm-up time, probing technique, and the light-and-beep result indicators.
- JII. “Diamond Tester Instruction Manual.” Documents the 6F22 9V battery requirement and the Diamond mode switch.
- Presidium. “How Does a Diamond Tester Work.” Explains the thermal and electrical conductivity principles behind tester readings.