This is a heavily pyritized Pleuroceras ammonite fossil collected near Forcheim, Germany. The fossil is approximately 185 million years old, from the Jurassic (the Pliensbachian stage). Naturally the color is much duller but these specimens have been brushed with a wire brush to create a brilliant gold shine.
Ammonites are a group of extinct animals which roamed the planet’s oceans for almost 250 million years. Ammonites are excellent index fossils, and it is often possible to link the rock layer in which a particular species or genus is found to specific geological time periods. They are also some of the most spectacular fossils, and you can find them quite commonly throughout the world (especially in limestones). This particular specimen was pyritized. Organisms are pyritized when they are in marine sediments saturated with iron sulfides. In other words, pyrite has replaced its limestone shell.