The best thing to do is take a look before you buy it.
Good advice!!
And have another look before it is tipped off the lorry.
My daughter went to have a look at what a supplier had to offer and chose the one she wanted and arranged for its delivery.
It was delivered when she was at work and it turned out not to be the same quality as she had seen and ordered.
So be warned some companies use double standards!
To answer your question a few stones up to the size of a bantams egg are OK but as I mentioned above you might get fist sized stones and that not so good.
So my advice is look at it before it is tipped!!
Sorry for the synicism but once bitten twice shy is my motto!