Getting gunk off of a seashell

I have a seashell about the size of a small/medium bowl that has been flattened out a little. On the inside, there is this beautiful mother-of-pearl look and is so pretty. However, on the outside, there is all this hardened gunk--don't know what to call it. But, in spots, I can see the mother-of-pearl on the top as well. How do I get this "stuff" off without damaging the shell?
  5 answers
Your comment...