Boil all you want, the rocks won't explode, that's a myth that's been floating around for a while.
The thing that could cause a rock to explode is baking one with moisture inside it. In that situation, the moisture converts to steam, causing the rock to explode. This won't happen by boiling it, because the temperature is controlled by the water.
It's the same reason popcorn explodes from a little kernal into a great big puff - it's the moisture in the grain converting to steam at high temps.
Also the same reason eggs explode in the microwave, but not when you boil them.
Anyways, as harmless as boiling rocks is, like PlantedRich, I don't see it being worth the effort. I'd probably just soak them a bit and scrub them with a brush.