sedimentary
Devil's Toenail (Fossil Oyster)
Gryphaea arcuata (Calcium Carbonate, CaCO3)
Hardness: 3 (calcite shell), Color: Dull grey to tan/brown due to mineralization, Luster: Dull/Earthy, Crystal Structure: Trigonal (mineralized calcite), Cleavage: None (conchoidal fracture when broken)
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Physical properties
Hardness: 3 (calcite shell), Color: Dull grey to tan/brown due to mineralization, Luster: Dull/Earthy, Crystal Structure: Trigonal (mineralized calcite), Cleavage: None (conchoidal fracture when broken)
Formation & geological history
Formed in shallow marine environments during the Jurassic and Cretaceous periods (approx. 200 to 66 million years ago). The specimen is a fossilized bivalve mollusk that lived on muddy sea floors.
Uses & applications
Primarily used for geological education, fossil collecting, and historical folk medicine. In modern times, they are popular entry-level items for fossil enthusiasts.
Geological facts
In British folklore, these were believed to be the toenails of the devil. They were also used in ancient medicine; ground into powder and mixed with water, they were thought to cure joint pains and itchy skin.
Field identification & locations
Identify by its thick, heavily curved left valve and much smaller flat right valve. Commonly found in Mesozoic clay and limestone deposits, particularly in Western Europe and parts of North America. Look for the distinctive 'hooked' shape similar to a claw.
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