"Is it a diamond?" used to have one test. Today a jeweller or buyer is really asking three separate questions: is it diamond or a simulant such as moissanite, is it natural or laboratory-grown, and how sure can I be without sending it to a lab? Each question needs a different kind of instrument. This guide explains which tester answers which question, and why the familiar pen tester is no longer enough.
Three questions, three kinds of tester
| Question | Instrument | How it works |
|---|---|---|
| Diamond or glass, CZ and most simulants? | Thermal diamond tester | Measures how fast the stone carries heat away from a heated tip |
| Diamond or moissanite? | Combined diamond and moissanite tester | Adds an electrical conductivity test to the thermal one |
| Natural or laboratory-grown? | Lab-grown (CVD/HPHT) diamond screener | Checks the diamond type, usually with light or spectroscopy, and refers possible lab-grown stones |
Why a thermal tester cannot spot lab-grown diamonds
A thermal tester relies on the fact that diamond conducts heat far better than glass or cubic zirconia. A laboratory-grown diamond is not an imitation: it has the same crystal structure as natural diamond, so it conducts heat the same way and reads as diamond every time. That is correct, as far as it goes; it simply does not answer the question most buyers now need answered.
Moissanite: why it fools some testers
Synthetic moissanite (silicon carbide) has been sold as a diamond substitute since the late 1990s. GIA reported at the time that its thermal properties are close enough to diamond's for it to pass as diamond on a thermal probe, and that electrical conductivity is what separates it. Most moissanite conducts electricity; most diamond does not. That is why combined testers measure both.
One complication explains a common complaint, "my lab-grown diamond tested as moissanite". Diamonds that contain traces of boron conduct electricity. Some laboratory-grown diamonds contain boron, so a combined tester can flag them as moissanite. A "moissanite" result on a stone that otherwise looks and behaves like diamond should be treated as a referral, not a verdict.
How lab-grown diamond screeners work
Most screeners do not look for "lab-grown" directly. They classify the diamond's type. Most natural diamonds are type Ia, which contain nitrogen in a form the screener can detect. Most colourless laboratory-grown diamonds, CVD and HPHT alike, are type II. So a screener passes type Ia stones as natural and refers type II stones for further testing.
"Refer" does not mean "lab-grown". Some natural diamonds are type II as well, and they will be referred too. A referred stone needs a gemmological laboratory or a more advanced instrument for the final answer. GIA has also noted that some screeners can miss certain lab-grown stones, which is another reason a screener is a filter, not a certificate.
Loose stones, mounted stones and melee
Before comparing models, settle three things:
- Loose or mounted. Some screeners take loose stones only; others have a probe for set jewellery.
- Stone size. Melee, the small stones in pavé and cluster settings, is where lab-grown mixing is hardest to see and most likely to be missed. Check the minimum size an instrument accepts.
- Volume. A showroom checking a few customer pieces needs something different from a manufacturer screening parcels of melee every day.
Testers in our range
Our diamond and gemstone tools range includes thermal and gem testers and laboratory-grown diamond screeners from established makers, including Presidium, SmartPro, Alrosa, DRC and J-Certain, alongside loupes, microscopes and carat scales. Most of these instruments are made by third parties; we supply and support them, and we will tell you plainly who makes what.
Our catalogue lists these instruments by name only, so we do not publish what each one detects here. Tell us whether you test loose or mounted stones, the smallest size you handle and your daily volume, and we will recommend an instrument and confirm its capabilities in writing.
Where to see and buy it
These are the Semsons Tech machines for this job. The store links let you book online; the other links open the full specifications or a quotation request.
| Machine and where to buy | Key facts |
|---|---|
| Diamond and CVD testers Full specifications Ask for a quote | Thermal, moissanite and lab-grown diamond screeners in our range Tell us the stones and sizes you test |
Setting laboratory-grown stones in production? See our guide to wax setting and casting lab-grown diamond jewellery. Questions about testers: send an enquiry or call +91 98216 12340.