http://planforpandemic.com/viewtopic.ph ... highlight=
Another tidbit from the SAS book - seemed really obvious after I read it, but I figure if I thought to myself "oh yeah..." then someone else might be interested too...
Avoid placing wet or porous rocks/stones near fires, especially if they've been submerged in water. Avoid using slate/soft rocks - those that crack, flake or sound hollow. When heated, if they contain moisture, that moisture will expand and the stone could explode or create airborn fragments that could injure someone nearby the fire.

And don't assume outside brick can be used - it'll explode... it holds moisture and while it appears dry, it's got moisture inside.