sedimentary
Ammonite Fossil
Ammonoidea (primarily composed of Calcite, CaCO3, or Pyrite)
Hardness: 3.5-4 (Mohs scale); Color: Gray, brown, or earthy tones (sometimes iridescent); Luster: Dull to sub-metallic; Structure: Planispiral coiled shell with visible suture lines; Specific Gravity: 2.7
- Hardness
- 3
- Color
- Gray, brown, or earthy tones (sometimes iridescent)
- Luster
- Dull to sub-metallic
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Physical properties
Hardness: 3.5-4 (Mohs scale); Color: Gray, brown, or earthy tones (sometimes iridescent); Luster: Dull to sub-metallic; Structure: Planispiral coiled shell with visible suture lines; Specific Gravity: 2.7
Formation & geological history
Formed through the process of permineralization or replacement within marine sedimentary environments (shale, limestone, or mudstone). These cephalopods lived from the Devonian period through the Cretaceous (approx. 400 to 66 million years ago).
Uses & applications
Primary use in educational collections, fossil hunting, and jewelry (especially when opalized). Large, well-preserved slabs are used as decorative architectural elements or high-end interior design focal points.
Geological facts
Ammonites are extinct marine mollusks closely related to modern chambered nautiluses and squids. Their name is derived from the Egyptian god Ammon, who was depicted wearing ram's horns, which look similar to the fossil's coiled shape.
Field identification & locations
Identify by looking for the distinct 'sidebar' or 'ram's horn' spiral. In the field, look for ribbing patterns and suture lines where internal walls met the outer shell. Commonly found in coastal cliffs or limestone quarries where ancient seas once existed.
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