Well, an isolated miniature black hole will evaporate very quickly - disastrously so, if it's fairly massive (like, say, the mass of a mountain released as energy all at once).

But I'm not so sure about what happens if the microhole is in an environment where it can draw in new material. My guess would be, if it's able to pull in mass faster than it's evaporating, it could remain stable... and if its infall rate is greater than the evaporation rate, it would grow.

But is the Earth's atmosphere dense enough to meet this criterion for a "stable" microhole? Is even solid rock dense enough? Is neutronium? I doubt it, but I don't know how to calculate it. After all, the event horizon of such a hole is teensy... much smaller than the radius of a proton... so it would always be "a long way" from any matter, even if it was located inside an atomic nucleus.

Except... it would have halacious tides, and might be able to draw in nuclei from much, much farther away than its event horizon.

Gulp!

I actually used this idea in a science fiction novel I wrote (sorry, it's unpublished). My microhole started out as 400 tons of lead. Things went downhill from there...

But I made some assumptions about the hole's infall radius that I can't really justify.