you cant boil rocks becasue any moisture trapped inside will expand putting it at risk for exploding
The only time I've seen this is when rocks are thrown into a fire.
Rocks that are fully submerged in boiling water shouldn't "explode"
A lot of people actually recommend boiling rocks to sterilize them after they've been in a tank with sick fish. I boiled some rocks I got out of a creek just to make sure no parasites or anything would live.