sedimentary

Coquina

Biogenic Limestone (Coquina)

AI-generated identification · may be incorrect

Hardness: 3 (calcite matrix); Color: Tan, beige, or white; Luster: Dull/earthy; Structure: Clastic, composed of shell fragments; Cleavage: None (fragmental); Specific Gravity: 2.3-2.5.

Hardness: 3 (calcite matrix)
Color: Tan, beige, or white
Luster: Dull/earthy

Identified July 19, 2026More sedimentary →

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Physical properties

Hardness: 3 (calcite matrix); Color: Tan, beige, or white; Luster: Dull/earthy; Structure: Clastic, composed of shell fragments; Cleavage: None (fragmental); Specific Gravity: 2.3-2.5.

Formation & geological history

Formed in high-energy marine environments (beaches) where waves break up shells and corals, which then accumulate and are lightly cemented together by calcium carbonate. Most deposits are relatively young (Pleistocene).

Uses & applications

Historically used as a building stone for forts and houses because its porous nature absorbs cannonball impacts without shattering. Today used for landscaping, road base, and concrete aggregate.

Geological facts

The Castillo de San Marcos in St. Augustine, Florida, is the most famous structure built from coquina; it proved bulletproof and cannonball-resistant during 18th-century sieges.

Field identification & locations

Identify by its 'oatmeal-like' appearance and visible shell fragments. Common in coastal regions, particularly Florida and the southeastern US. Reacts strongly (fizzes) to dilute hydrochloric acid.