How to Tell If a Diamond Is Real

Is this a diamond? And if so, is it natural or lab-grown? These are separate questions with different answers. The first can often be settled quickly by a jeweller. The second needs a gemmological laboratory, because a lab-grown diamond is diamond in every physical sense.

You will find these recommended everywhere. They are fun, but none of them is reliable:

The fog test

Breathe on the stone. Diamond disperses heat quickly, so the mist clears almost at once, while glass and cubic zirconia stay fogged a little longer. But moissanite clears just as fast, and a greasy stone can confuse the result.

The water test

Drop a loose stone in water and see if it sinks. Diamond is dense and sinks, but so do most simulants. It only rules out very light imitations, and it cannot be done with a set stone.

The newspaper test

Place a loose stone face down on print. A well-cut diamond bends light so strongly you should not be able to read through it. Some simulants behave the same way, and a poorly cut diamond may not.

The scratch test

Never do this. Diamond can scratch glass, but so can other hard stones, and a diamond can chip if struck at the wrong angle.

At best, these tests suggest a stone might be an obvious fake. None can confirm a diamond, and none can tell you whether it is natural or lab-grown.

A trained eye with a loupe catches many simulants straight away. Cubic zirconia and glass often show rounded, worn facet edges and a slightly different look to their sparkle. Moissanite shows doubled facet edges when viewed at an angle, because it is doubly refractive.

Electronic testers confirm the impression. A thermal tester checks how quickly the stone conducts heat; a combined tester also checks electrical conductivity, which separates diamond from moissanite. These take seconds and do not harm the stone.

Standard testers identify both natural and lab-grown diamonds as diamond. Telling them apart takes specialist laboratory equipment that reads the tiny differences in how the crystals grew.

The simplest way to know is the grading report. Reputable laboratories state the origin clearly, and the report number is usually laser-inscribed on the edge of the stone, invisible to the eye but readable under magnification, so the diamond and its paperwork can be matched.

Buy any significant diamond with an independent grading report from a respected laboratory, and check that the inscription on the stone matches the number on the report. The hallmark inside a ring confirms the metal, not the stone.

If you already own a ring and want to know what is in it, any experienced jeweller can test it quickly. For certainty about origin, a laboratory report is the final word.