sedimentary

Coquina / Fossiliferous Limestone

Coquina (composed primarily of Calcium Carbonate / CaCO3)

Hardness: 2-3 (Mohs scale), Color: Cream, tan, or grayish-white, Luster: Dull or earthy, Crystal structure: Microcrystalline calcite within shell fragments, Cleavage: None (fragmental), Specific gravity: 2.0-2.4

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in high-energy marine environments where waves accumulate broken shells, coral, and other organic debris. Over time, these are weakly cemented together by calcite. Typically found in deposits from the Quaternary to recent geological ages.

Uses & applications

Historically used as a building material for fortresses (due to its ability to absorb cannon fire), road base, landscaping stone, and as a source for lime.

Geological facts

The Castillo de San Marcos in Florida is famously built from coquina; the stone was so porous that it swallowed cannonballs rather than shattering. It is essentially a 'natural concrete' made of biological leftovers.

Field identification & locations

Identify in the field by its distinct 'oatmeal' or 'shredded wheat' appearance where individual shell bits are visible. Commonly found along coastal areas in Florida, North Carolina, and Australia. Reacts vigorously (bubbles) when touched with dilute hydrochloric acid or vinegar.

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