sedimentary

Coquina

Bioclastic Limestone

Hardness: 3 (Mohs scale), Color: tan, white, or gray, Luster: dull to earthy, Crystal Structure: amorphous mixture of shell fragments, Cleavage: none, Specific Gravity: 2.3-2.6

Identified May 10, 2026More sedimentary →

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Physical properties

Hardness: 3 (Mohs scale), Color: tan, white, or gray, Luster: dull to earthy, Crystal Structure: amorphous mixture of shell fragments, Cleavage: none, Specific Gravity: 2.3-2.6

Formation & geological history

Formed in high-energy marine environments where shell fragments and coral debris accumulate and are cemented together by calcium carbonate. Most deposits are relatively young geologically, typically from the Pleistocene or Holocene epochs.

Uses & applications

Used primarily as a construction material for building blocks and road base. Historically significant for fort construction due to its ability to absorb cannonball impacts. Also used as decorative landscaping stone.

Geological facts

The Castillo de San Marcos in St. Augustine, Florida, was built using coquina. Its porous nature allowed it to absorb the impact of cannonballs rather than shattering, making it an incredibly effective defensive material.

Field identification & locations

Identify in the field by looking for a 'cereal bar' texture made entirely of visible shell fragments. It will react and fizz strongly when in contact with dilute hydrochloric acid (vinegar may also produce a weak reaction). Common along the coastlines of Florida and the Carolinas.