Fossiliferous Limestone (Coquina / Shell Limestone)

Physical Properties

Hardness: 3-4 (Mohs scale); Color: Rusty brown, tan, and white; Luster: Dull to earthy; Structure: Accumulation of visible shell fragments (bioclasts); Specific Gravity: 2.3-2.7.

Formation & Geological History

Formed in high-energy marine or coastal environments where shell debris (mollusks, brachiopods) accumulates rapidly and is cemented together by calcite. Often found in strata from the Paleozoic to Cenozoic eras.

Uses & Applications

Used primarily in building construction (historically for forts and walls), as road base material, in the manufacturing of cement, or as decorative garden stones.

Geological Facts

Famous structures like the Castillo de San Marcos in Florida were built from coquina; the stone's porous nature allowed it to absorb cannonball impacts rather than shattering.

Field Identification & Locations

Identify by a 'fizz' test (reacts with dilute hydrochloric acid) and the presence of visible, often broken shell fragments. Common in coastal regions and ancient seafloor deposits.

Identified on: 4/24/2026

Mode: Standard