Bricks can explode if exposed to extreme heat or if there's a freak air bubble in the brick somewhere that you can't see. No wood burning fire smaller than your house isn't going to break a brick and it's a 1/10,000 chance to get that freak defect and 30x that for the defect to be dramatic enough to cause harm. Especially if you're getting commercial bricks made in a factory in with QC standards. Most of this applies to natural rocks. Less chance of voids for some reasons, more reason for voids in other reasons. Basically, if the rock is lighter than you expected when you picked it up, maybe don't use it or go digging for a void in the rock with a drill. Answer from JudgeHoltman on reddit.com