"Turritella Agate" from the Eocene of Wyoming, USA. (5.7 cm across along the base)

Of all the molluscs, the gastropods (snails) have made the most ecological adaptations. They can be found in almost all fundamental environments: marine, freshwater, terrestrial. Most gastropods live in the ocean, and have a single, asymmetrically coiled, external shell of calcium carbonate (CaCO3 - usually aragonite). The hard calcareous shell is the most easily fossilized part of the gastropod. The soft parts of a snail (the “slug” portion) include a well developed head having eyes, tentacles, and a mouth, and a well developed, strong, muscular foot used principally for locomotion. The shell is carried upright on the snail’s back, or is partially dragged behind. When threatened by a predator, many snails can retract their soft parts into the shell’s interior for protection.

Many fossil snails in the Paleozoic rock record are often not well preserved, or are preserved as internal molds. The original aragonite of many gastropod shells is not stable on geologic time scales, and often recrystallizes or dissolves completely away. Fossil snail shells in Mesozoic and Cenozoic rocks are usually better preserved.

The rock shown above is lacustrine fossiliferous chert packed with fossil snails. This material is popular with rockhounds and lapidarists, who call it "Turritella Agate". Well, it's not agate - it's fossiliferous chert. And the snails aren't Turritella, they are Elimia tenera (Animalia, Mollusca, Gastropoda, Cerithioidea, Pleuroceridae).

Stratigraphy & age: Green River Formation, Lower Eocene