First off, send that piece to me. Not Rock. Lol! It's a nice spade or Celt or what ever tool it is, and more pics of it would be nice. The black spots are moss or fungi. I see it often on pieces that I find in wet areas. I never really purposely remove it, but removal doesn't affect the patina. If the idea of a fungus bothers you, scrape the spots off. If not, let their be a fungus among us!