Ammonite Fossil PairMade up of mostly open pockets, this ammonite pair has a dark brown color with three black rows of sea beds. The back is mostly polished with a brown color and white waves. Ammonite fossils have been found on every continent, in all sorts of sizes and in lots of colors, though the most common naturally occurring colors for these fossils are brown and gray. Ammonite fossils form when ammonoids died and their shells became embedded in sand or silt. The shell was protected from damage over time due to the forming layers. As the layers formed and created pressure on the shell, mineral-rich water would seep into the chambers and eventually crystallize into the rock-like fossil we find today. Ammonite can sometimes end up with a rainbow-like sheen on the surface, opalized or pyritized.