sedimentary
Ammonite Fossil
Ammonoidea (primarily composed of Calcite, CaCO3)
Hardness: 3-4 (Calcite replacement); Color: Yellow, brown, tan, or white; Luster: Vitreous to pearly; Structure: Spiral planispiral chambers; Cleavage: None (fossilized structure)
- Hardness
- 3-4 (Calcite replacement)
- Color
- Yellow, brown, tan, or white
- Luster
- Vitreous to pearly
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Physical properties
Hardness: 3-4 (Calcite replacement); Color: Yellow, brown, tan, or white; Luster: Vitreous to pearly; Structure: Spiral planispiral chambers; Cleavage: None (fossilized structure)
Formation & geological history
Formed through permineralization of extinct marine cephalopods in marine sedimentary environments during the Devonian to Cretaceous periods (approx. 400 to 66 million years ago).
Uses & applications
Used primarily as decorative collector specimens, educational material, and in jewelry (pendants and cabochons).
Geological facts
Ammonites are more closely related to living squid and octopuses than to the nautilus they resemble. Large specimens can grow up to several feet in diameter.
Field identification & locations
Identified by the distinct coiled shell divided into septa (internal walls). Commonly found in sedimentary outcrops, particularly in Madagascar, Morocco, and the UK. Look for the suture patterns on the exterior.
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