fossil

Fossil ammonite or gastropod cast

Ammonoidea or fossil gastropod; exact identification requires clearer details

AI-generated identification · may be incorrect

A brown, weathered fossilized shell or internal cast with a rounded, dark central bulge and rough sedimentary matrix. The material is likely calcite-replaced shell in limestone or mudstone; hardness about 3–4, dull to earthy luster, with no visible crystal structure or cleavage.

Identified August 27, 2026More fossil →

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Physical properties

Formation & geological history

Formed when a marine mollusk shell was buried in sediment and later mineralized or preserved as a mold/cast, commonly during the Paleozoic–Mesozoic eras (about 541–66 million years ago).

Uses & applications

Primarily a collecting, educational, and decorative specimen; ammonites and large gastropod fossils can have modest commercial value.

Geological facts

The visible form may be an internal mold rather than the original shell, with erosion removing much of the surrounding matrix and ornamentation.

Field identification & locations

Look for spiral coiling, chamber partitions, or a smooth aperture to distinguish ammonites from gastropods; dilute acid may fizz if calcitic, but testing can damage it. Similar fossils occur in marine limestone and shale worldwide.