sedimentary

Fossilized Oyster (Gryphaea or Exogyra)

Crassostrea / Gryphaea (Bivalvia)

Hardness: 3-4 (calcite); Color: Tan, beige, grey, and cream; Luster: Dull to pearly; Structure: Asymmetrical bivalve with thick, concentrically layered calcite shells; Cleavage: None (fractures shell-like).

Hardness: 3-4 (calcite)
Color: Tan, beige, grey, and cream
Luster: Dull to pearly

Identified June 29, 2026More sedimentary →

Explore Fossilized Oyster (Gryphaea or Exogyra) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through permineralization of ancient marine mollusks in shallow seas, typically during the Cretaceous or Jurassic periods (66-200 million years ago) within sedimentary layers like limestone or marl.

Uses & applications

Primarily used as educational specimens, collectibles, and in evolutionary paleontology studies. Historically used in poultry feed as a calcium source.

Geological facts

C. virginica and its ancestors were reef-builders. Some fossil oysters, known as 'Devil's Toenails' (Gryphaea), are famous for their extremely curved lower shells.

Field identification & locations

Identify by looking for distinct growth rings and thick, heavy shell fragments compared to modern shells. Common in dry creek beds or chalk cliffs where ancient oceans once existed.

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