Ammonite fossil, sliced in half and each side polished.  This piece is simply stunning, with crystallization within the chambers, lots of suture patterns on the back, and a few flecks of opalization.

Ammonites lived during the Devonian-Cretaceous period.  An ancient form of cephalopod, they are related to the modern nautilus, and share the same golden ratio spiral formed as they grow larger.

This pair is 5.5" at the widest point.