How Does a Diamond Tester Work?

A handheld diamond tester is, at heart, a heated metal tip and a meter. You touch the tip to a clean facet and the instrument reads how quickly the heat drains out of the tip and into the stone. Diamond carries heat away faster than glass, cubic zirconia or any of the older imitations, so the meter climbs to “diamond” for a diamond and stays low for the rest.

That is the whole of it. The tester asks a single question — does this stone move heat the way diamond does? — and answers in seconds. It does not look at the stone, weigh it, grade it or know where it came from, and every mistake a tester makes comes from a stone that happens to share that one property.

Synthetic moissanite — silicon carbide — was the first imitation to beat the heat test. When the Gemological Institute of America examined it in 1997, as it was about to reach the jewellery trade, every synthetic moissanite in the study registered as “diamond” on every thermal tester the researchers used. Its thermal properties are simply too close to diamond's for the method to tell them apart.

So a heat-only tester can clear a stone of being glass or cubic zirconia. It cannot clear it of being moissanite, which is much closer to diamond in look and heft than the imitations the test was designed for.

The same study named the properties that do separate them. Moissanite is doubly refractive, so the edges of the back facets look doubled when you look into the stone — at an angle, under a loupe, not straight down through the table, where the doubling usually does not show. It is also lighter: a specific gravity of 3.22 against diamond's 3.52, enough for moissanite to float in a heavy liquid that diamond sinks in.

Testers sold as multi-testers or moissanite testers add a second reading: whether a small electric current passes through the stone. Most colourless diamonds do not conduct and much moissanite does, so a stone that reads “diamond” on heat and also conducts is flagged as moissanite.

It is the better instrument, but the electrical half is reading an indication, not an identity. Of the 23 synthetic moissanites in GIA's study, 13 conducted at all — ten did not — and the authors concluded that electrical conductivity should never be treated as diagnostic, because some diamonds conduct as well.

The diamonds that conduct are the ones that hold boron, which gemologists class as type IIb. Boron makes both natural and laboratory-grown type IIb diamonds conduct electricity, and no other type of diamond does. Natural type IIb stones are rare and usually blue or grey. Laboratory-grown ones are not: GIA reports that more than 80% of the HPHT-grown diamonds on the D-to-Z colour scale submitted to it since 2020 carry detectable boron. A stone like that conducts — exactly the signal a multi-tester reads as moissanite.

A laboratory-grown diamond is diamond: the same carbon in the same crystal, carrying heat the same way. A thermal tester reads it as diamond because it is one, and an electrical check agrees unless the stone holds boron — in which case it may be called moissanite, which is wrong in the other direction. Nothing a handheld tester measures differs between a mined diamond and a grown one; what separates them is their growth history, which leaves traces only advanced instruments can read.

Desktop screening devices such as GIA's iD100 read those traces with spectroscopy and return “Pass” for a natural diamond or “Refer” for a stone that needs further testing. A refer is not a verdict. GIA's 2024 review of laboratory-grown diamonds notes that referred stones may be natural or grown, that some screening devices cannot refer imitations at all, and that no rapid, inexpensive tool definitively identifies every laboratory-grown diamond. The last word belongs to a gemological laboratory.

Every wrong reading has the same cause: the stone shares the property the instrument measures. Read that way, the failures are predictable rather than mysterious, and each one has a check that catches it.

Each instrument settles a narrower or wider question, and the right one depends on what is at stake. For most people the order is the same: a tester first, because it is quick and inexpensive, and a laboratory only when the stone is worth the fee or its origin matters.

No. A diamond tester works by touching the stone and measuring heat, and in its better form electricity. A phone has no probe and can measure neither, so an app sold as a “diamond tester” is doing something else: reading a photograph.

That is what our own Diamond Identifier does, and it is worth being exact about what it buys you. From a photograph it estimates the four Cs and gives a value range — a first look at what a piece appears to be, loose or set. It cannot confirm that a stone is a diamond, it cannot tell moissanite from diamond, and it cannot tell laboratory-grown from mined: the questions a tester and a laboratory exist to answer.

Used in that order, the two are complementary rather than competing. The app tells you whether a piece is worth taking to a tester; the tester tells you whether it is a diamond; a laboratory, if the stakes justify it, tells you the rest.