sedimentary
Iridescent Ammonite Fossil
Ammonoidea (subclass); Composition: Aragonite (Calcium Carbonate, CaCO3) often replaced by Calcite or Pyrite
Hardness: 3.5-4.0 (Mohs scale); Color: Brownish-grey with rainbow iridescence (purple, green, orange); Luster: Pearly to iridescent (ammolite-like), dull where unpolished; Crystal Structure: Orthorhombic (aragonite shell); Cleavage: None (conchoidal fracture).
- Hardness
- 3
- Luster
- Pearly to iridescent (ammolite-like), dull where unpolished
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Physical properties
Hardness: 3.5-4.0 (Mohs scale); Color: Brownish-grey with rainbow iridescence (purple, green, orange); Luster: Pearly to iridescent (ammolite-like), dull where unpolished; Crystal Structure: Orthorhombic (aragonite shell); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through permineralization of extinct marine cephalopod shells in sedimentary basins during the Jurassic or Cretaceous periods (approx. 66–200 million years ago). The iridescent layer is caused by the preservation of the original nacreous (mother-of-pearl) layers of the shell.
Uses & applications
Used primarily as decorative collector specimens, educational fossils, and in the jewelry industry (high-quality iridescent segments are sometimes sold as 'Ammolite' gems).
Geological facts
Ammonites are more closely related to living octopuses and squids than to the Chambered Nautilus they resemble. The iridescence is a structural color caused by light interference as it passes through thin, stacked layers of aragonite platelets.
Field identification & locations
Identify in the field by the planispiral (flat-coiled) shell shape and distinctive 'suture' patterns on the surface. Commonly found in Madagascar, Morocco, and the Alberta Bearpaw Formation. Collectors look for unbroken spirals and vibrant color flashes across the surface.
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