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)
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Physical properties
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).
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.
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