Coquina

Physical Properties

Hardness: 2-3 (Mohs scale); Color: Tan, brownish-white, or gray; Luster: Dull to earthy; Texture: Highly porous and coarse; Composition: Almost entirely of shell fragments (Calcium Carbonate, CaCO3); Specific Gravity: Relatively low due to porosity.

Formation & Geological History

Formed in high-energy marine environments like shorelines or barrier beaches where waves break shells into fragments. Over time, these fragments are lightly cemented together by calcite. Most deposits are relatively young, dating from the Pleistocene to Holocene epochs.

Uses & Applications

Historically used as a primary building material in Florida and the Caribbean because its soft structure absorbs cannonball impacts without shattering. Today, it is used for decorative landscaping, road base material (paving), and as a component in concrete.

Geological Facts

The Castillo de San Marcos in St. Augustine, Florida, the oldest masonry fort in the continental US, is built from coquina. It was indestructible against 18th-century cannons because the rock's porous nature allowed it to swallow the cannonballs rather than crumble.

Field Identification & Locations

Identify in the field by looking for a 'cereal bar' or 'oatmeal' appearance made of visible tiny seashells or shell grit. It will fizz vigorously when exposed to diluted hydrochloric acid or vinegar due to the high calcium carbonate content.

Identified on: 4/27/2026

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