Neapolitan pizza in charcoal barbecue
The bubbles in Neapolitan pizza come from air, emitted by live yeast in the pizza dough, expanding when it's headed up.

While bubbles can be created by baking the pizza on a stone, it's not required.  You'll also get bubbles by just rubbing some oil on the grill grates and placing the pizza dough directly on the grill.  The key to getting bubbles is having well-made pizza dough that includes lots of air bubbles and baking it under high heat (500F to 900F).

If you do use a pizza stone on the grill,


check the packaging for the pizza stone to ensure it can handle the heat you plan to subject it to
pre-heat the stone on the grill before putting the pizza dough on it.  this can take a long time.