fossil

Fossilized Oyster Shell

Exogyra or Ostrea (Gryphaeidae/Ostreidae family)

Hardness: 3.5-4 Mohs; Color: Tan, brown, gray; Luster: Pearly to dull/earthy; Structure: Asymmetrical, calcified bivalve shell with concentric growth rings; Cleavage: None.

Hardness: 3
Color: Tan, brown, gray
Luster: Pearly to dull/earthy

Identified July 4, 2026More fossil →

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Physical properties

Hardness: 3.5-4 Mohs; Color: Tan, brown, gray; Luster: Pearly to dull/earthy; Structure: Asymmetrical, calcified bivalve shell with concentric growth rings; Cleavage: None.

Formation & geological history

Formed during the Cretaceous or Jurassic periods (65-200 million years ago) through the mineralization of marine bivalve shells in shallow sea environments. The calcium carbonate was replaced by minerals like calcite or silica over geological time.

Uses & applications

Primarily used for geological study, paleontological collecting, and educational purposes. Some fossiliferous limestone containing these shells is used in building materials.

Geological facts

Often called 'devil's toenails' in certain varieties (like Gryphaea), these fossils indicate that the region where they were found was once part of a prehistoric shallow ocean floor.

Field identification & locations

Identified by the distinct curved, cupped shape and thick, layered calcium shell. Commonly found in sedimentary rock formations such as limestone and shale in regions like Texas and the UK.

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