i have personally seen rocks explode forcefully in a fire, but they were smallish acorn-sized ones taken right out of a stream and placed right in the middle of the fire. even at such a small size it was awfully dangerous (but perhaps smaller ones are a much greater risk than big ones that can't fly too far). i would expect that a lot of heat would be needed to make a big one blow up--since it's much more difficult to heat the entire thing rapidly, cracking is more likely than exploding. nevertheless, it definitely is possible for rocks to explode and shoot hot shards everywhere from a fire.