I am going to assume you put the rocks on the rack where the plates and such would usually go, right?
As long as you didn't impede the movement of the rotary spraybar or block and vent holes/drains/etc then washing the rocks will have absolutely nothing to do with breaking your dishwasher.
The item being washed doesn't move around in a wash cycle, so why would washing the rocks break the machine??
Take a moment to think about it logically, ie how the machine actually works, rather than by assumption and you will realise that blaming the rocks makes no sense. You wouldn't blame the plates if this happened washing them would you?????
Saying that, when you use thw arranty or insurance to get this fixed do not tell them you were washing rocks. Even though it will have done nothing to damage the machine they will say you used it beyond normal service and void the insurance or warranty.