Coquina

Physical Properties

Hardness (Mohs scale): 3 (variable); Color: Tan, white, grey, or brownish; Luster: Dull/Earthy; Structure: Clastic, composed of visible shell fragments; Cleavage: None; Specific Gravity: 2.3-2.6.

Formation & Geological History

Formed in high-energy marine environments (beaches or near-shore) where wave action breaks shells into fragments that are later cemented together by calcium carbonate. Most deposits are relatively young (Cenozoic era).

Uses & Applications

Used as a historical building material (e.g., Castillo de San Marcos in Florida), for road base, in landscape architecture, and as a source of calcium for animal feed.

Geological Facts

Coquina is famous for its unique ability to absorb cannon fire rather than shattering, making it an excellent fortification material in Florida’s history.

Field Identification & Locations

Identify in the field by looking for a rock that looks like a compressed pile of oatmeal or small shell bits. It often fizzes vigorously when in contact with dilute hydrochloric acid.

Identified on: 4/18/2026

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