Essup, It's called cultured stone, you will need to put up some 15# tar paper and some lath using roof nails, when you put up scratch coat if you just let it dry a couple hours before you start installing stone it seemed to stick alot better,and what really made the stones stick well was mixing in some thin set with the type n or s morter mix, using rubber gloves and spreading the scratch coat and the morter on the stones by hand and rubbing it in real good was twice or more faster, and stayed on well. Till I did this I would put up 5 stones and one would fall and knock 3 off.
My guess is that you were mixing the mud too dry. When doing brick work, you want the mix to be fairly dry, and I've seen allot of peopld do the same with cultured stone. Then the rock falls off and the mortar doesn't have any "stick" to it. If you mix it up wet, the rock sticks really well. There is a point of too much water, but that's pretty easy to figure out.
