sedimentary
Ammonite Fossil
Ammonoidea (subclass)
Hardness: 3.5–4 (Mohs); Color: Brown, tan, reddish-orange with intricate, fern-like suture patterns; Luster: Pearly to earthy; Structure: Planispiral coil (spiral); Composition: Primarily calcite, aragonite, or silica replacing original shell.
- Hardness
- 3
- Luster
- Pearly to earthy
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Physical properties
Hardness: 3.5–4 (Mohs); Color: Brown, tan, reddish-orange with intricate, fern-like suture patterns; Luster: Pearly to earthy; Structure: Planispiral coil (spiral); Composition: Primarily calcite, aragonite, or silica replacing original shell.
Formation & geological history
Formed through the fossilization of extinct marine cephalopods during the Devonian through Cretaceous periods (approx. 400 to 66 million years ago). The remains were buried in seafloor sediment and mineralized over millennia.
Uses & applications
Majorly used as decorative display specimens, jewelry (especially iridescent 'Ammolite' pieces), and as index fossils for dating geological layers.
Geological facts
The intricate 'lace' patterns on the surface are called suture lines, marking where the internal chamber walls (septa) met the outer shell. They were relatives of the modern nautilus and octopus.
Field identification & locations
Identify by its distinct spiral shape and complex branching suture lines. Common in sedimentary basins worldwide, notably Madagascar, Morocco, and Canada. Collectors look for clarity of sutures and surface iridescence.
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