sedimentary

Coquina

Coquina (Bioclastic Limestone)

Hardness: 2-3 (Mohs scale); Color: Cream, tan, or yellowish-brown; Luster: Dull to earthy; Texture: Coarse-grained and clastic; Structure: Composed of poorly cemented shells and shell fragments; Porosity: Exceptionally high.

Hardness: 2-3 (Mohs scale)
Color: Cream, tan, or yellowish-brown
Luster: Dull to earthy

Identified April 10, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in high-energy marine environments (beaches or offshore bars) where wave action accumulates and sorts shells. Over time, calcium carbonate from the shells acts as a weak cement. Typically found in Quaternary or Tertiary deposits.

Uses & applications

Historically used as building material (notably in Florida and the Caribbean), road base, and for landscaping. Its soft nature makes it uniquely capable of absorbing shock from cannonballs, which assisted in fortification defenses.

Geological facts

Castillo de San Marcos in St. Augustine, Florida, is the oldest masonry fort in the continental US and was built using coquina. While it is technically a limestone, it is unique because the individual shells are often still clearly visible and identifiable.

Field identification & locations

Identify in the field by its distinct 'oatmeal' or 'shredded wheat' appearance and the obvious presence of shells. It will effervesce (fizz) vigorously when in contact with diluted hydrochloric acid or vinegar due to the calcium carbonate content.