Coquina or Fossiliferous Limestone

Physical Properties

Hardness: 3 (Mohs scale); Color: Cream, tan, or grayish-white; Luster: Dull to earthy; Crystal structure: Hexagonal (rhombohedral) as calcite; Cleavage: Perfect rhombohedral in calcite components; Specific gravity: 2.7.

Formation & Geological History

Formed in high-energy marine environments where shell fragments and coral debris accumulate. Over time, these are compacted and cemented by calcium carbonate. Most terrestrial coquina deposits are from the Pleistocene or Holocene epochs.

Uses & Applications

Historically used as a building stone (notable in Florida and Spain). Currently used in road base, landscaping, and as a source of lime for agriculture.

Geological Facts

The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; its porous nature allowed it to absorb cannonball impacts rather than shattering. The name comes from the Spanish word for 'cockle' or 'shellfish'.

Field Identification & Locations

Identify in the field by looking for visible shell fragments or fossils and checking for a reaction (fizzing) when exposed to dilute hydrochloric acid. Common along coastal regions of Florida and Australia.

Identified on: 4/26/2026

Mode: Standard