What glue would you use to glue rock to rock and have it water proof?

I would use an outdoor epoxy

I agree, a two part epoxy glue should keep 'em together.

Premium PL. construction adhesive

I'd use liquid nails. You buy it from Lowes and load the tube like caulk in your caulk gun. Then you squeeze some out on one rock and adhere the other rock to it. How much you squeeze out is depends on how much surface you want to adhere to the other rock.

E6000 glue. Wait at least 24 hours for it to cure. It comes in clear or white or black. I usually use the clear.

Thanks again for chiming in. I have already tried the E6000 didn't work, rocks and glue never bonded even after washing rocks in alcoho. Liquid nail hadn't thought of that one thanks.

two part epoxy for outdoor