Please help. I am not sure what this is. We moved into our rental about a month ago and these dark spots were not here. I just noticed them today. We have not put anything on the hearth and the fireplace is electric. The only thing that has gone over the hearth is the Roomba Vacuum. I am also not sure what type of stone the fireplace surround is. I tried dawn soap and hot water with a sponge and I tried a magic eraser and it did not lighten them at all. Any ideas on how to get rid of these spots?