Ammonite

 An extinct subclass of cephalopods, whose present-day descendants include he octopus, squid & nautilus.  Ammonites lived from approximately 400 to 65 million years ago (late cretaceous).  Their well-preserved fossilized shells are found today, and are often cut & polished.  Small ammonites are often used in jewelry design, while larger pieces are prized as display keepsakes.