Can you tell a diamond is real by breathing on it?

The test reads thermal conductivity. Warm, moist air condenses on a cool stone, and a material with high thermal conductivity disperses that heat fast enough that the fog clears within a second or two. Diamond is the most thermally conductive transparent material in normal life, which is why the test works against glass and cubic zirconia: both conduct heat poorly and stay foggy for several seconds. The test breaks the moment moissanite is in play. Moissanite's thermal conductivity sits close enough to diamond that fog clears at the same speed, which is why the test only filters obvious fakes.

The fog test, the water test (a real diamond sinks fast in a glass of water) and reading newsprint through the stone all filter glass and cubic zirconia reasonably well. None of them separate diamond from moissanite, which is now the most common simulant. A 10x loupe is more useful: a real diamond shows sharp, crisp facet edges, while moissanite shows a faint doubling along facet lines viewed from the side. For anything important like a vintage piece or an unbranded second-hand ring, a jeweller's thermal-and-electrical tester gives a confident answer in under a minute.