Ammonites are fossils of once living creatures that lived some 66 to 400 million years ago during the Paleozoic and Mesozoic Eras, and disappeared with the extinction of the dinosaurs more than 66 million years ago. Ammonites were closely related to octopuses and squid, except they lived in a shell. Their shells were made of aragonite, a shiny iridescent material, the same material pearls are made of.
Fossilized ammonites were formed when the remains of the ammonite were buried by sediment millions of years ago. Over time, the chambers of the buried shell filled with rich mineral deposits, and after being exposed to high temperatures and pressures over millions of years the chemical composition changed and the ammonite became fossilized. As many as 10,000 species may have existed, leaving an incredibly diverse variety of fossils behind.
Each ammonite is unique due to the way they were fossilized under the earth for millions of years and because of the vast number of different species. Ammonites are the perfect example of geology and gemology coming together, which is why ammonites are one of my favorites.