sedimentary

Coquina

Biogenic Limestone (Calcium Carbonate, CaCO3)

AI-generated identification · may be incorrect

Hardness: 2-3 (Mohs scale); Color: Cream, tan, or white with brown shell fragments; Luster: Dull/Earthy; Crystal Structure: Amorphous/Bioclastic; Cleavage: None; Specific Gravity: 2.5-2.7

Hardness: 2-3 (Mohs scale)
Color: Cream, tan, or white with brown shell fragments
Luster: Dull/Earthy

Identified June 6, 2026More sedimentary →

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Physical properties

Hardness: 2-3 (Mohs scale); Color: Cream, tan, or white with brown shell fragments; Luster: Dull/Earthy; Crystal Structure: Amorphous/Bioclastic; Cleavage: None; Specific Gravity: 2.5-2.7

Formation & geological history

Formed in high-energy marine environments where shell fragments and calcified remains of marine organisms accumulate and are cemented together by calcium carbonate. Most deposits are relatively young, dating from the Pleistocene to Holocene epochs.

Uses & applications

Used primarily as a construction material (especially in coastal areas like Florida), road gravel, and historically for building fortifications due to its ability to absorb cannonball impacts.

Geological facts

The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; its porous nature meant that instead of shattering, the walls would 'swallow' cannonballs shot at it.

Field identification & locations

Identify by its 'oatmeal-like' appearance and visible shell fragments. It will effervesce (fizz) vigorously when exposed to dilute hydrochloric acid or vinegar. Found commonly along coastal zones.