How to Tell If a Diamond Is Real

Most guides list twenty tests and tell you nothing about which ones are worth doing. The honest answer is that a handful genuinely narrow things down, several prove almost nothing, and a couple risk wrecking the stone you are trying to check.

Here is the whole list, sorted by how much a result should actually change your mind.

The two tests worth doing

The read-through test. Put the loose stone flat side down on a page of newsprint and look straight down through it. A real diamond refracts light so severely that you will not see letters, not even smeared ones. If you can read the text or make out grey shapes, you are almost certainly looking at glass or cubic zirconia. This is the closest thing to a decisive home test, and it is completely useless once a stone is set in a ring, because you need to look through the flat table.

A loupe and ten seconds. A jeweller's loupe costs about ten dollars and tells you more than every other test combined. Real diamonds nearly always have something in them: a tiny feather, a pinpoint, a hint of cloud. A stone that is perfectly clean under magnification and came cheap is a warning, not a bargain. While you are in there, look at the facet edges. If every edge appears doubled, you are looking at moissanite.

The tests that prove less than people think

The water test. A diamond has a density of 3.52, so it sinks. So does cubic zirconia, which is heavier still, and so does glass. All it rules out is a hollow fake or plastic, which you would have spotted anyway.

The fog test. Better than its reputation. Diamond conducts heat away so efficiently that a breath of condensation clears in about a second, where glass and CZ hold it for two or three. It is a real physical difference and you can see it. It just will not separate a diamond from moissanite, which also sheds heat quickly.

Two tests to skip entirely

Heating a stone with a lighter and dropping it in cold water is repeated everywhere and it is a bad idea. Thermal shock can crack a real diamond along an existing inclusion, and you will have destroyed the thing you were trying to verify.

Scratching glass proves your stone is harder than 5.5 on the Mohs scale. Quartz, topaz, sapphire, CZ and moissanite all clear that bar. You learn nothing and risk chipping a girdle.

The stone that beats every home test

Moissanite is the reason home testing has a ceiling. It sits at 9.25 on the Mohs scale, it sinks, it sheds heat, it is tougher than CZ, and on the thermal testers that jewellers used for decades it lights up as diamond. The old giveaway is that it has more fire than a diamond does, throwing rainbow flashes where a diamond throws white light, and under a loupe its double refraction makes facet edges look doubled.

If you suspect moissanite, that is the point where a home test stops being useful and a jeweller with a proper moissanite tester takes about four seconds to settle it.

Frequently asked questions

Worth being clear: everything here, and the app itself, is an educational reference. At-home tests narrow things down, they do not certify a stone. Before you buy, sell or insure anything of real value, get it looked at by a certified gemologist or an independent appraiser.

Go deeper on one test