sedimentary

Coquina

Coquina (Limestone variant)

Hardness: 2-3 (Mohs scale), Color: tan, cream, off-white, Luster: earthy to pearly, Crystal structure: Trigonal (as calcite fossils), Cleavage: None (conchoidal fracture at micro-scale), Specific Gravity: 1.2 - 2.0

Identified June 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in high-energy marine environments where waves accumulate seashells, coral, and shell fragments. Over time, these fragments are naturally cemented together by calcite. Mostly found in deposits from the Quaternary to modern periods.

Uses & applications

Used historically as building material (notably Castillo de San Marcos), landscape decoration, source of calcium for agricultural lime, and in some construction aggregates.

Geological facts

Because it is composed of shell fragments, Coquina was used to build fortifications; its porous nature would absorb cannonball impacts rather than shattering like brick or harder stone. The word comes from the Spanish for 'cockle' or 'shellfish'.

Field identification & locations

Identify by its visible shell-like texture and its reaction (fizzing) when exposed to a weak acid like vinegar. Found along coastal regions, particularly in Florida, the Carolinas, and Mexico.