Ammonites are fossils of shelled creatures similar to a nautilus that lived 250 to 350 million years ago. These animals could be very small or larger across than a human. They became fossils after being buried in mud or ash and their shells were slowly replaced by minerals, in this case agate. The simple beauty of their natural spiral shells evokes timelessness and continuity. This handmade sterling silver necklace is on an 18 inch chain.