Hi, I have never had trouble with sidewalk chalk. Quite often I do wash it off, but I have left it on and returned after the next rain and didn't see any evidence of it remaining. I try to use chalk that closely matchs the color of the stone. Gray chalk on limestone is hardly noticed, but evens out the weathering discolorations. I do prefer to use a mirror or a powerful light. Some cemeteries paint their tombstones white. I have seen tombstones painted white, silver, blue and green. I'm not sure how that may affect them. Paul