sedimentary

Ammonite Fossil

Ammonoidea (typically preserved as Calcite CaCO3 or Pyrite FeS2)

Hardness: 3.5-4 (calcite version); Color: Tan, brown, or grey; Luster: Dull/earthy (unpolished); Structure: Planispiral coiled shell with visible sutures/ribs; Cleavage: None (conchoidal fracture when broken).

Hardness: 3
Color: Tan, brown, or grey
Luster: Dull/earthy (unpolished)

Identified February 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments through permineralization or cast/mold processes. These cephalopods lived from the Devonian period (400 million years ago) until the end of the Cretaceous (66 million years ago).

Uses & applications

Predominantly used as index fossils for dating geological layers, educational specimens, jewelry (when polished or iridescent), and home decor.

Geological facts

Ammonites are more closely related to modern octopuses and squids than to the nautilus they resemble. Large species could reach over 2 meters in diameter.

Field identification & locations

Identify by the characteristic spiral shape and 'ribbed' texture. Commonly found in limestone or shale beds worldwide, particularly in Morocco, Madagascar, and parts of the UK and USA. Collectors should look for clear suture lines and preservation quality.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Sandstone

Arenite (primarily SiO2)

sedimentary