You can clean most with a tooth brush. Some rocks crumble when you pick them up. Obviously they cannot be cleaned this way. Really, a soft piece of material, a rock blanket or a toothbrush. Some rocks will react with water, some with air. It really is an impossible question to ask until you identify the rock. Some will discolor with exposure to light or the heat from your skin or the oils in your skin. For those that do not, water and a small brush will clean most surface dirt from your rocks.