This is an exceptional Ammonite fossil which still has its exterior ridged shell intact except for one small piece as pictured. When looked at closely you can determine from this small break that if the exterior shell was removed this Ammonite will almost certainly display a vibrant red iridescence, an exceptional collectors piece.
Ammonites are an extinct group of marine invertebrate in the subclass Ammonoidea of the class Cephalopoda. The name "ammonite", from which the scientific term is derived, was inspired by the spiral shape of their fossilized shells, which somewhat resemble tightly coiled rams horns. These molluscs are more closely related to living coleoids (i.e. octopus, squid and cuttlefish) than they are to shelled nautiloids such as the living Nautilus species. The earliest Ammonites appear during the mid Devonian age roughly 350 million years ago, and the last species died out during the Cretaceous-Paleogene extinction event 66 million years ago.