There is a good way but it's very long winded and would take far too long to explain here ,but the basic idea is to create maybe 4 or 5 stones and then create a grid of particles and intance the 5 stones onto them using a random expression and then finally creating an exression for random scale and oriantation.
As you can see this can get quite involved for a new user so i am afraid you may be stuck with creating tileable patch of random stones and then duplicating them of course you do run the risk of being able to see the pattern but it does depend on your camera angle so you may be ok.