What a totally cool idea to use river rocks as towel hooks. I have used robe hooks as towels holders, but these are so much cooler. Especially if you are using river stones anywhere in you bath, ie shower walls or floors. They’d also be nice outside by a
DIY Rock Knobs for Cabinets or Doors (Handles, pulls) - just attach screw with epoxy, then add a couple washers on the rock side to make it stick out a bit from the surface and tighten on with a locking nut.