Rock Identifier
Ammonite fossil in matrix (Fossilized ammonoid cephalopod; likely an ammonite such as Dactylioceras or a related Jurassic taxon) — fossil
fossil

Ammonite fossil in matrix

Fossilized ammonoid cephalopod; likely an ammonite such as Dactylioceras or a related Jurassic taxon

AI-generated identification · may be incorrect

The specimen shows a planispiral, ribbed shell preserved in dark gray rock matrix, with possible mineral replacement or coating. Original shell calcite/aragonite is commonly recrystallized; Mohs hardness is roughly 3–4 for calcitic material and about 6–7 for silicate matrix.

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Physical properties

The specimen shows a planispiral, ribbed shell preserved in dark gray rock matrix, with possible mineral replacement or coating. Original shell calcite/aragonite is commonly recrystallized; Mohs hardness is roughly 3–4 for calcitic material and about 6–7 for silicate matrix.

Formation & geological history

Ammonites were marine cephalopods that lived from the Devonian to the end-Cretaceous, Devonian–Cretaceous (~409–66 million years ago). After burial in marine sediment, the shell was mineralized and later exposed by weathering or preparation.

Uses & applications

Primarily a fossil-collecting and educational specimen; polished ammonites are also used decoratively and occasionally in jewelry. Value depends heavily on species, completeness, preservation, and preparation quality.

Geological facts

The spiral reflects chambered shell growth, while internal septa helped regulate buoyancy. The visible dark matrix and metallic-looking patches may represent pyrite, calcite, or weathering products.

Field identification & locations

Identify by the coiled, regularly ribbed shell and chamber pattern; common sources include Jurassic rocks of the United Kingdom, France, Germany, Morocco, Madagascar, and Canada. Estimated value: $10–$75 for a small partial specimen, or $100–$500+ if unusually complete, attractive, or rare.

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