Just remember your basic geology. Igneous stones simply cannot absorb moisture like a friable sedimetnary or metamorphic rock. Gather stones away from water sources and give a visual inspection for cracks. A fire ring is not a surefire barrier to spreading fire. If you ignite subsurface roots the embers can travel literally for months before igniting in a surface fire. A fire ring merely acts to physically corral your fuel. Your best option is to invest in a fireblanket to lay the fuel on. This also keeps the soil from being sterilised by heat. Fire rings should be dismantled and the blackened surfaces turned inward, unless the campground is well established with repeat use by others.