What is the breath test for sapphires?

So, the breath test is this plain, non-destructive field thing where you exhale pretty hard onto a gemstone and then watch how quickly the fog fades. On a genuine sapphire, the condensation tends to disappear almost at once, usually within 1–2 seconds, because the stone has high thermal conductivity.

With glass or plastic imitations, the warmth behaves a bit differently, and that means the fog hangs around for a few more seconds before it clears. It can be useful while you are scanning a market, but it is not really a final scientific check. Also, it cannot tell apart a natural sapphire from a lab-made synthetic sapphire, because both end up having the same thermal properties.