sedimentary

Coquina / Fossiliferous Limestone

Calcite-rich Bioclastic Sedimentary Rock (CaCO3)

Hardness: 3 (Mohs scale); Color: Cream, tan, or reddish-brown (from iron oxidation); Luster: Dull/Earthy; Structure: Clastic fossil fragments poorly cemented; Cleavage: None (fractures irregularly); Specific Gravity: 1.2 to 2.4 (highly porous).

Hardness: 3 (Mohs scale)
Color: Cream, tan, or reddish-brown (from iron oxidation)
Luster: Dull/Earthy

Identified March 11, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in high-energy marine environments like beaches and barrier islands where shell fragments accumulate and are naturally cemented by calcium carbonate from seawater. Most common in the Cenozoic era.

Uses & applications

Historically used as a construction material for forts (like Castillo de San Marcos) because it absorbs cannonball impacts without shattering. Today it is used in landscaping, road base, and specimen collecting.

Geological facts

Because it is composed of loosely cemented shell fragments, it is one of the only rocks that can 'absorb' shocks. During the Siege of St. Augustine in 1702, cannonballs reportedly stuck in the coquina walls rather than breaking them.

Field identification & locations

Identify in the field by its visible shell fragments and rough, porous texture; it will also react/fizz when a drop of vinegar (weak acid) is applied. Found abundantly in coastal regions like Florida and North Carolina.

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