How to do it
Hold the stone between two fingers, or hold the ring by the band. Bring it close to your mouth and breathe on it the way you would fog up a pair of glasses. Watch the surface.
A real diamond will clear in about a second, sometimes so fast it is hard to catch the fog forming at all. Glass, cubic zirconia and most simulants stay misted for two to four seconds. Once you have seen both, the difference is obvious without counting.
Why it works
Diamond is the best thermal conductor of any natural material on earth, roughly five times better than copper. The heat and moisture in your breath get pulled into the stone and spread out almost instantly, so the condensation has nothing to sit on.
Cubic zirconia is a thermal insulator by comparison. The moisture sits on a cold surface and takes its time evaporating. That gap in conductivity is enormous, which is why this test works at all despite being so crude.
| Stone | Thermal behaviour | Fog clears in |
|---|---|---|
| Diamond | Exceptional conductor | Around 1 second |
| Moissanite | Very good conductor | Around 1 second |
| Cubic zirconia | Poor conductor | 2 to 4 seconds |
| Glass | Poor conductor | 3 to 5 seconds |
| White sapphire | Moderate | 2 to 3 seconds |
Three things that ruin the result
A dirty stone. Skin oil and soap film hold moisture on the surface and will make a real diamond look slow. Wash it with warm water and a drop of dish soap, dry it, and let it sit for a minute before testing.
A cold stone. A ring that has been outside in winter starts well below room temperature and fogs heavily regardless of what it is. Let it warm up first.
Testing twice too quickly. Your first breath adds heat. The second reading is always muddier. Wait half a minute.
What a pass actually means
If the fog clears instantly, you have ruled out glass, cubic zirconia and most cheap simulants. That is genuinely useful, because those are what the overwhelming majority of fakes are made of.
What you have not ruled out is moissanite, which behaves almost identically here. If the stone is unmounted, follow up with the read-through test. If it is set, look at the facet edges under a loupe: moissanite doubles them, diamond does not.
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
How long should the fog last on a real diamond?
Does the fog test work on a mounted ring?
Why does it fail on moissanite?
Can I do it twice in a row?
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.