sedimentary

Coquina

Biogenic Limestone (Calcite, CaCO3)

Hardness: 3 on Mohs scale; Color: Tan, beige, or white; Luster: Dull/Earthy; Structure: Clastic/Bioclastic with visible shell fragments; Porosity: High; Specific Gravity: ~2.7.

Hardness: 3 on Mohs scale
Color: Tan, beige, or white
Luster: Dull/Earthy

Identified March 20, 2026More sedimentary →

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Physical properties

Hardness: 3 on Mohs scale; Color: Tan, beige, or white; Luster: Dull/Earthy; Structure: Clastic/Bioclastic with visible shell fragments; Porosity: High; Specific Gravity: ~2.7.

Formation & geological history

Formed in high-energy marine environments (beaches and barrier islands) where shells are broken into fragments by wave action and naturally cemented together by calcium carbonate over hundreds to thousands of years.

Uses & applications

Used as a building material in coastal areas, as road base ballast, and in landscaping or decorative masonry for its unique texture.

Geological facts

The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; the material's porous nature allowed it to absorb cannonball impacts rather than shattering, making the fort nearly indestructible.

Field identification & locations

Identify in the field by looking for a 'cereal bar' texture made of small, crushed seashells. It will fizz when exposed to weak acid (dilute HCl or vinegar). Commonly found along the coasts of Florida, North Carolina, and Australia.