sedimentary
Fossilized Oyster (Devil's Toenail)
Gryphaea (extinct genus of bivalve mollusks)
Hardness: 3-4 (calcitic); Color: Light tan, grey, to cream; Luster: Dull to earthy; Shape: Characteristic coiled, thick shell with growth lines; Composition: Calcium carbonate (calcite).
- Hardness
- 3-4 (calcitic)
- Color
- Light tan, grey, to cream
- Luster
- Dull to earthy
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Physical properties
Hardness: 3-4 (calcitic); Color: Light tan, grey, to cream; Luster: Dull to earthy; Shape: Characteristic coiled, thick shell with growth lines; Composition: Calcium carbonate (calcite).
Formation & geological history
Formed through the fossilization of marine bivalves in shallow sea beds. These specimens typically date from the Triassic, Jurassic, or Cretaceous periods (approx. 66 to 250 million years ago).
Uses & applications
Primarily used for educational study, geological research, and as a collector's specimen. Historically used in folk medicine (as 'Devil's Toenails').
Geological facts
The genus Gryphaea is an index fossil used by geologists to date rock layers. They evolved such a thick, heavy shell to sit stabilized on the muddy sea floor without sinking or being flipped by currents.
Field identification & locations
Identify by its distinctive 'claws-like' or 'toenail' shape and prominent concentric growth rings. Commonly found in limestone or clay deposits in the UK, Europe, and North America.
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