This is your luck day, Bern. You don't have to do anything. Actually, a sealer can cause you a lot more trouble than leaving your fireplace alone. A little soot on the outside is kind of a matter of taste. Some people actually like the soot since it shows that the fireplace is actually used and not just for decoration. Others get concerned by the "dirt" but your stone is easy to clean. Tri-sodium phosphate (TSP) or something like Soilax will get rid of it easily. Be sure that you rinse well. The water will not hurt the stone for a million years or so.