The test as it is usually described
Fill a glass about three quarters with room temperature water. Drop the loose stone in. A real diamond sinks quickly to the bottom. A fake, so the story goes, floats or drifts down slowly.
The physics behind it is sound as far as it goes. Diamond has a specific gravity of 3.52, which means it is three and a half times denser than water and there is no ambiguity about which way it is going.
Why it barely narrows anything down
Look at what else sinks. Every common diamond simulant is denser than water, and cubic zirconia, the most widespread fake of all, is substantially denser than diamond itself.
| Material | Specific gravity | In a glass of water |
|---|---|---|
| Cubic zirconia | 5.6 to 6.0 | Sinks, fast |
| Diamond | 3.52 | Sinks |
| Moissanite | 3.22 | Sinks |
| White sapphire | 4.0 | Sinks |
| Glass | 2.4 to 2.8 | Sinks |
| Water | 1.00 | - |
Everything in that table sinks. A test where every possible answer gives the same result is not a test. All you have genuinely excluded is plastic and hollow imitations, and those give themselves away the moment you pick them up.
The version that does work
Specific gravity itself is a genuinely decisive property. The numbers above are far enough apart that measuring one settles the question outright. Jewellers do it by weighing the stone in air, then weighing it suspended in water, and dividing.
That needs a scale accurate to a hundredth of a carat and a proper suspension rig, which is why it lives in a workshop and not on a kitchen counter. But it is the reason the water idea persists: there is a real, powerful test buried under a useless folk version of it.
Spend your time on these instead
If the stone is loose, the read-through test takes ten seconds and actually separates diamond from glass and CZ. If it is mounted, the fog test works on a set stone and rests on a genuine difference in thermal conductivity.
Not sure what you are looking at?
Photograph the stone and the app gives you a visual read on the 4Cs, an estimated value range, and what to check next. Free on iOS and Android.
Frequently asked questions
Do real diamonds sink in water?
What does float then?
Is there a version of this test that works?
Can I do the water test on a ring?
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.