Nothing wrong with boiling rocks.
That's a myth born from overly concerned folks passing on info they don't quite understand, but mean well.
Rocks only explode if they are wet, and then are heated well beyond boiling temp - like a soaking wet rock thrown into a fire (and this is only some rocks, it's perfectly safe to do it with something like granite, because granite won't really absorb water like sandstone or something might), the water expands into steam, and the pressure increase fractures the rock, sometimes violently. Water->Steam involves something over a 1000 fold pressure/volume increase, whereas room temp gas->boiling gas only involves about a doubling of pressure/volume.
I'd give the rock a good scrubbing with a decent stiff-bristeled brush to remove any dirt/etc. If you are overly concerned about introducing something alive, then give it a bleach soak or boil it.
ETA: it looks like there is some striation in the rock, so possibly more of a gneiss? (still safe to boil...)
Last edited by lochaber; 01-23-2015 at 09:07 PM.
Reason: looked closer at pic...