Clean the stonework thoroughly using a sponge, dish soap and water. Scrub any difficult to remove stains with a scrubbing brush. Check the condition of the stone making up both the fireplace and the fireplace surround. Determine the type of stone used in the fireplace, as this will affect the kind of repairs that are possible.
Sand any scratched stones using low grit sandpaper, to remove the surface damage. This will make the stones look newer than before and can make them stand out. If necessary, consider sanding down other stones to maintain the overall look of the fireplace.
Mix up a small amount of the masonry repair poultice appropriate for the kind of stone you are using. Follow the manufacturer's instructions. Apply the poultice to the larger cracks and gouges in the stones, using a grouting tool. Allow the poultice time to set. Sand the poultice down using a low grit sandpaper to blend with the surface of the stones.
Chip out the mortar surrounding the stones using a hammer and cold chisel. Slide the stone out of the fireplace. Prop up the gap with wooden blocks to preserve the integrity of the fireplace. Cut or order a new block of stone of the same material. Prepare fresh mortar. Remove the wooden blocks. Apply mortar to the sides of the hole with a trowel. Slide the new stone into place. Fill the gaps around the edges with more mortar.