Rock Identifier
Fossilized Oyster Shell (Gryphaea or Exogyra (Family Ostreidae)) — fossil
fossil

Fossilized Oyster Shell

Gryphaea or Exogyra (Family Ostreidae)

Hardness: 3.5 to 4 on the Mohs scale (calcite/aragonite composition); Color: Cream, beige, brownish-white; Luster: Dull to pearly; Crystal structure: Trigonal (calcite); Specific gravity: 2.5 to 2.7

Hardness
3
Color
Cream, beige, brownish-white
Luster
Dull to pearly
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Physical properties

Hardness: 3.5 to 4 on the Mohs scale (calcite/aragonite composition); Color: Cream, beige, brownish-white; Luster: Dull to pearly; Crystal structure: Trigonal (calcite); Specific gravity: 2.5 to 2.7

Formation & geological history

Formed in marine environments during the Mesozoic and Cenozoic eras (ranging from roughly 65 to 200 million years ago) through the fossilization and permineralization of ancient mollusk shells buried in sediment.

Uses & applications

Commonly collected by fossil enthusiasts, used in educational displays, and sometimes studied by geologists for paleoenvironmental reconstruction.

Geological facts

Colloquially known as 'devil's toenails' in certain regions due to their curved, claw-like appearance, these fossils are excellent index fossils for specific marine strata.

Field identification & locations

Easily identified by their distinctly curved, asymmetrical shell shape with pronounced growth ridges and rough, ribbed exterior surfaces. Frequently found in limestone and sedimentary rock formations worldwide, particularly in areas previously covered by ancient shallow seas.

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