Rare is relative as to the site and the state or country. It's a fossilized ammonite, and a very, very nice complete one. (Jealous). I have never found one, just a couple little pieces from Delaware over the years. Yours is a beauty, yes, it has value but I don't have info on that. Best bet is to try to find a similar sold one on Ebay or Etsy or such.
Yeah it's an ammonite. It has been worked with cutting tools, however, to remove it from the host rock, and clean it up. Here is a finished example, and you can see the similar beveled shaping and material removal.