Does moissanite pass a diamond tester?

A thermal tester measures how fast heat drains from the probe tip into the stone. Diamond conducts heat exceptionally well, which is what the tester is really checking, and moissanite happens to conduct heat well too, which is why it fools that generation of device. Dual testers were built specifically to close that gap by adding an electrical conductivity check.

Handheld testers misread more often than people expect. A cold room, a stone fresh off a window sill, a probe touching the metal setting instead of the stone, or a very small stone can all produce a false result in either direction, and some lab-grown diamonds with trace boron conduct slightly and can confuse a cheap electrical check. Test at room temperature, on a clean dry stone, probe flat on the table facet, and repeat the reading before believing it.

And never scratch-test a stone. The old advice about scratching glass proves nothing, because moissanite, sapphire and plenty of imitations all scratch glass, and dragging a stone across anything risks chipping a girdle edge you then pay to repair. If a result genuinely matters, a certificate or a gemmologist is cheap compared with the doubt.

Tester behaviour is a property of the material, not a verdict on quality: moissanite passing a thermal tester does not make it a diamond, and it does not make it a scam either, provided it was sold as moissanite. Buy the stone that was advertised, with paperwork that matches. Lab-grown diamonds read as diamond on every tester because they are diamond, which is one reason buyers who care about that certainty lean lab-grown. If you are weighing that exact choice, our comparison covers sparkle, price and care side by side.