If there's any moisture trapped in the rocks (more common than you might think), it can boil and build up a pressurized gas pocket when heated; this can then cause the rock to burst. So, not like a fireball kind of explosion, but still a dangerous occurrence.
Tedzbug- I suggest trying to find a local aquarists' club. They would no doubt be happy to share some disease-free filter media with you, and are a handy resource in general.
Tedzbug- I suggest trying to find a local aquarists' club. They would no doubt be happy to share some disease-free filter media with you, and are a handy resource in general.