Sedimentary

Ammonite Fossil (Polished Half / Cleoniceras)

Ammonoidea (Subclass), often Cleoniceras genus; fossilized with Calcite (CaCO3) and Aragonite

AI-generated identification · may be incorrect

Hardness: 3.5-4 (calcite chambers). Color: Honey-yellow, amber, brown, and translucent. Luster: Vitreous to pearly. Structure: Spiral fluting with internal chambers (septa). Cleavage: None (conchoidal fracture when broken).

Hardness: 3
Color: Honey-yellow, amber, brown, and translucent
Luster: Vitreous to pearly

Identified March 3, 2026More sedimentary →

Explore Ammonite Fossil (Polished Half / Cleoniceras) in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Formed through permineralization of marine cephalopod shells. These specimens typically date back to the Cretaceous period (approx. 110 million years ago), found in marine sedimentary layers.

Uses & applications

Commonly used in jewelry, as decorative display pieces, for geological education, and by fossil collectors.

Geological facts

Ammonites are extinct relatives of the modern nautilus and octopus. The name 'Ammonite' comes from the Egyptian god Ammon, who was depicted with rams' horns that match the shell's shape. This specimen has been cut and polished to reveal the internal chambers used for buoyancy.

Field identification & locations

Identify by its 'phi' or logarithmic spiral and internal chamber walls. Found globally, but those with yellow-calcite-filled chambers are very common in Madagascar. Collectors should look for clear, well-defined septa and a lack of air bubbles (which might indicate a resin fake).