Coquina

Physical Properties

Hardness: 3 (Mohs scale); Color: tan, buff, or cream; Luster: dull to earthy; Crystal Structure: amorphous shells within calcite matrix; Specific Gravity: 2.1-2.4

Formation & Geological History

Formed in high-energy marine environments (beaches) where shell fragments are accumulated by waves and naturally cemented by calcite. Most deposits are from the Pleistocene or Holocene epochs.

Uses & Applications

Used primarily as building stone (historically), road base material, and source of lime for agriculture.

Geological Facts

The Castillo de San Marcos in St. Augustine, Florida, was built from coquina; the stone's porous nature allowed it to absorb cannonball impacts rather than shattering.

Field Identification & Locations

Identify by its distinctive 'oatmeal' or 'granola' appearance made of visible shell fragments. Reacts strongly (fizzes) with cold dilute hydrochloric acid.

Identified on: 5/3/2026

Mode: Standard