fossil

Polished ammonite fossil

Ammonoidea; likely a polished, chambered ammonite such as Acanthoceras or related Cretaceous form

AI-generated identification · may be incorrect

Fossilized aragonitic shell, commonly replaced by calcite or silica; Mohs hardness about 3–4, dark brown to black matrix with cream spiral chambers, subdued waxy-to-glassy polish, and strong planispiral symmetry. The visible septa and sutures are characteristic of ammonites.

Identified August 9, 2026More fossil →

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Physical properties

Formation & geological history

Ammonites were extinct marine cephalopods that lived from the Devonian to the end-Cretaceous extinction, about Devonian–66 million years ago. Their shells were buried in marine sediments and mineralized during diagenesis, then cut and polished.

Uses & applications

Primarily used as decorative specimens, educational fossils, jewelry, and collectibles; not generally used in construction or industry.

Geological facts

The spiral records successive growth chambers, while complex sutures helped strengthen the shell against water pressure. Ammonites are especially prized from Madagascar, Morocco, England, Germany, and Canada.

Field identification & locations

Identify by the coiled, chambered shell and repeated septal partitions; polishing commonly exposes the internal spiral. Value depends on species, size, preservation, provenance, and polish quality.