sedimentary

Devil's Toenail

Gryphaea (extinct genus of foam oysters, typically composed of Calcite CaCO3)

Hardness: 3 (calcite); Color: Grey to brownish-grey; Luster: Dull to earthy; Shape: Extremely thick, curved lower shell (valve) resembling a claw or toenail; Cleavage: Not applicable for fossilized material; Structure: Massive/Biogenic.

Hardness: 3 (calcite)
Color: Grey to brownish-grey
Luster: Dull to earthy

Identified April 6, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed during the Jurassic and Cretaceous periods (approx. 200-66 million years ago). These were bivalve mollusks that lived on muddy sea floors in shallow marine environments. Over time, their calcium carbonate shells were buried in sediment and lithified.

Uses & applications

Primarily used for educational geological specimens, fossil collecting, and occasionally as index fossils to date sedimentary rock layers.

Geological facts

In folklore, these fossils were once believed to be the toenails of the devil. They are a classic example of evolution within the fossil record, showing a transition from life attached to the sea floor to living freely on the sediment.

Field identification & locations

Identify in the field by their distinctive hooked, 'C' shaped shell which is much thicker than modern oyster shells. They are commonly found in limestone or shale outcrops, particularly in the United Kingdom and the United States (Texas).

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