Hi Shayla:
As I recall, it was Lineco acid free EVA, but that was years ago, and now I can't find any EVA glues online.
PVA certainly would work but would be harder to scrub off the stones. Or maybe methyl cellulose would be a good choice. I'm not sure, but Klucel E might work, as well. It should stick, and I'm thinking you could make it thick enough to slightly fill the texture of the stones. It has viscosity of 250-800cps in 10% solution by weight in water, as opposed to Klucel G, which has viscosity of 125-450cps in 2% solution by weight in water.
I suggest doing what I did -- grab a few rocks out of the garden, wash them off, and see what sticks.