oh, and just keep the torch far away and keep it moving to prevent boiling/burning the wax. It’s a lot like retipping diamond prongs with the stone still in place. Heat slowly and keep it moving, then back off slowly and avoild rapid cooling (that’s when they’ll crack on you). Beware: heating stones will change the color on a lot of natural varieties. If there’s a chance of color change, go heat free.
I’ve also found that a small bit of boiling isn’t a dealbreaker; if you get the final bond nice and liquidy on a hot stone, any bubbles float away from the stone.
And please don’t try this on your rare, valuable rough! A lot can go wrong.