Robin, river bottom style rocks we have aplenty here. A word of caution though is hot rocks will break, sometimes explosively, if hit with cold water, though I’m sure with a little experimentation that a workable arrangement could be made. What this line of thinking reminds me of are the absolutely ingenious Shakers who gave themselves a lifestyle well above that of the general public. If any here have the chance to visit a Shaker museum property do so because they are chock full of things that might give you useful ideas. There is a great one in Hancock, MA which is between the NY border and Pittsfield, MA. They did a lot more than just make classic furniture.