After comparing five diamond testers across a mixed bag of natural stones, moissanite simulants, and cubic zirconia, I came away with strong opinions about which units earn their price. Cheap thermal pens still have a place for quick triage, but dual testers that combine heat and electrical conductivity catch the moissanite that fooled jewelers for years. This guide pulls the five most credible options on Amazon right now, ranked by accuracy on synthetics, build quality, and how often you trust the readout on the first try.
Quick comparison table
| Model | Test type | Best for |
|---|---|---|
| Presidium Gem Tester PGT | Thermal | Spot checks |
| Presidium DuoTester II | Dual (heat + electrical) | Moissanite separation |
| GIA iD100 | UV spectroscopy | Lab-grown ID |
| Mizar M5 Diamond Tester | Thermal | Beginners |
| HDE Diamond Selector II | Thermal | Budget |
1. Presidium Gem Tester PGT: trusted thermal workhorse for jewelers
The Presidium PGT is the thermal probe most jewelers keep in a drawer. It uses a fine retractable tip, calibrates against ambient temperature, and gives a clear LED bar reading within three seconds of contact. I found it accurate on every natural diamond above 0.10 carat and reliably flagged cubic zirconia. It cannot separate moissanite, which is its only real weakness. The PGT runs on a 9V battery for roughly 25 hours of testing, includes a built-in metal-alloy detector that prevents false readings against silver settings, and ships with a calibration sample stone.
2. Presidium DuoTester II: dual heat and electrical separates moissanite
The DuoTester II is what you want when synthetic stones are on the table. It pairs the same thermal probe used in the PGT with an electrical conductivity sensor, then cross-references both readings before flashing diamond, moissanite, or simulant. In my tests it caught every moissanite I tried, including a 2 carat that had been mistaken for a natural by two different pen testers. The unit is heavier than entry models, uses a 9V battery, and includes a polishing cloth. Auto shutoff trips at three minutes to save power.
3. GIA iD100: lab-grown separation through deep-UV spectroscopy
The GIA iD100 is the only consumer-priced unit that separates natural from lab-grown using deep-UV light absorption. It is overkill for casual buyers but indispensable for estate buyers and pawn brokers facing CVD and HPHT inventory. Press the stone against the read window, wait two seconds, and the screen reports either pass or refer for further testing. It runs on a rechargeable internal battery rated for around six hours of continuous use and connects to a PC for batch logging. At the price, this is a working tool, not a hobby gadget.
4. Mizar M5 Diamond Tester: easy entry point with clean ergonomics
The Mizar M5 is the unit I would hand a new bench worker. It runs on a single 9V battery, beeps on a positive diamond reading, and uses a five-LED ladder to show signal strength. The probe tip is replaceable, which matters because thermal pens are abused by drop tests and bent tips. It is thermal only, so moissanite will read positive, but for screening estate lots before deeper testing it gets the job done quickly. The grip rubber is grippy enough to use with cotton gloves on.
5. HDE Diamond Selector II: budget thermal pen for hobbyists
The HDE Diamond Selector II is the cheap entry tester that floods Amazon search results, and frankly it works for what it is. It uses a thermal probe and a colored LED strip, flags diamonds in roughly two seconds, and runs on a 9V battery. It will not separate moissanite, the build is plastic, and the tip is not replaceable. For a once-in-a-while check on a flea-market find or a gifted ring, it returns sensible answers. Do not rely on it for buying decisions over a few hundred dollars.
How to choose
Start by asking what you need to separate. If you are screening estate jewelry where lab-grown is unlikely, a thermal probe like the Presidium PGT or Mizar M5 handles 95 percent of casework. The moment moissanite enters the conversation, jump to the DuoTester II. It pays for itself the first time it catches a stone a pen tester missed.
Battery life matters more than it seems. Thermal probes use a 9V cell that drains faster than you expect when the unit sits powered on between tests. Buy two batteries and label one as backup. Replaceable tips also matter, because dropped probes bend, and a tester with a sealed tip is essentially disposable.
For appraisal-grade work that has to separate CVD and HPHT synthetics from natural diamonds, the GIA iD100 is the only consumer-priced answer. It costs more than the others combined, but for a working buyer it pays back fast. Hobbyists should stop at a dual tester and treat the iD100 as a future upgrade.
Frequently asked questions
Can a diamond detector tell moissanite from diamond?+
Only dual-tester models can. Thermal-only pens flag both as diamond because moissanite conducts heat similarly. The Presidium DuoTester and GIA iD100 use electrical conductivity to separate them in under two seconds.
Do these work on mounted stones?+
Yes, most use a flat tip that contacts the table facet. You can test stones in rings, pendants, and earrings, though tight prong settings may obstruct the probe.
Can they detect lab-grown diamonds?+
Standard thermal testers read lab-grown as natural because they share atomic structure. Only the GIA iD100 separates natural from synthetic via UV spectroscopy in the consumer price tier.
How accurate are budget detectors?+
They reliably flag cubic zirconia and glass but cannot separate moissanite. For pawn-shop spot checks, that is enough. For appraisal work, spend on a dual tester.