fossil

Polished ammonite fossil

Ammonoidea (likely a Jurassic or Cretaceous ammonite), preserved in limestone/mudstone

AI-generated identification · may be incorrect

Fossilized coiled cephalopod shell with visible chamber sutures and a polished dark-brown matrix; original shell is aragonite commonly replaced by calcite. Mohs hardness about 3–4 for the carbonate material; dull to vitreous polish, curved planispiral structure, and no prominent cleavage.

Identified August 9, 2026More fossil →

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Physical properties

Formation & geological history

Ammonites were marine cephalopods that lived from the Devonian to the end-Cretaceous extinction (~409–66 million years ago); this specimen is likely Mesozoic. Burial in marine sediment followed by mineral replacement and lithification preserved the shell.

Uses & applications

Primarily used for education, decoration, fossil collecting, and jewelry or cabochons when cut and polished. Commercial value depends on size, preservation, sutures, provenance, and polish.

Geological facts

The spiral records chamber growth, while the intricate sutures strengthened the shell against water pressure. Ammonites are extinct relatives of modern nautiluses, squid, and cuttlefish.

Field identification & locations

Identify by the tightly coiled planispiral shell, segmented chambers, and distinctive sutures; common sources include Madagascar, Morocco, England, Germany, and Canada. Avoid acids, which can etch carbonate surfaces.