sedimentary
Ammonite Fossil (Polished Section)
Ammonoidea (typically mineralized with Calcite CaCO3)
Hardness: 3 (calcite infill); Color: Brown, amber, and tan chambers with dark septa; Luster: Vitreous to pearly when polished; Structure: Planispiral coiled shell divided into internal chambers (phragmocone).
- Hardness
- 3 (calcite infill)
- Color
- Brown, amber, and tan chambers with dark septa
- Luster
- Vitreous to pearly when polished
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Physical properties
Hardness: 3 (calcite infill); Color: Brown, amber, and tan chambers with dark septa; Luster: Vitreous to pearly when polished; Structure: Planispiral coiled shell divided into internal chambers (phragmocone).
Formation & geological history
Formed through permineralization of extinct marine mollusks during the Devonian through Cretaceous periods (approx. 400 to 66 million years ago). Minerals in groundwater filled the empty shell chambers after burial in seafloor sediment.
Uses & applications
Primarily used as decorative display pieces, in jewelry (pendants), and as index fossils for geological dating. High value for private collectors and educational museums.
Geological facts
Ammonites are more closely related to modern octopuses and squids than to the Nautilus they resemble. Some specimens, known as Ammolite, develop a gemstone-quality iridescent play-of-color on their outer shell.
Field identification & locations
Identify in the field by looking for spiral ribbed patterns or 'suture' lines on the surface of limestone or shale. Common locations include the Jurassic Coast in the UK, Madagascar, and Morocco. Polished sections like this one clearly reveal the internal septa (chamber walls).
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