For what it's worth : my pa, who is a fossil man, marks all his rocks by
applying a square block of typing eraser (remember that, computer-istis?
Thick white paint with which you covered up bloopers on paper. Remember
paper?) to a non-essential portion of the specimen and then writes the data
on it with a Rotring .2mm pen. I have done the same on some rocks of my
own, and it seems nice and permanent. If you're really worried, add a layer
of clear nail varnish to seal it in.