Coquina

Coquina is a bioclastic limestone composed almost entirely of cemented fragments of shells, coral, and other marine skeletal debris. The name comes from the Spanish word for "cockle" or "shellfish."

It is typically poorly cemented, soft, and very porous, with the broken shells often clearly visible to the naked eye. Fresh coquina is soft enough to cut with hand tools, but it hardens on exposure to air.

Florida is famous for coquina; the 17th-century Castillo de San Marcos fortress in St. Augustine was built from it, and its springy, shell-rich stone famously absorbed cannon fire rather than shattering.

Coquina forms in high-energy shallow marine settings — beaches, sandbars, and shorelines — where wave and current action breaks up shells and concentrates the fragments into shell hash.

The accumulated shell debris is then cemented together by calcium carbonate precipitated from seawater or groundwater. Because cementation is often incomplete, coquina remains porous and relatively weak.

It forms most readily where shell-producing organisms (clams, mollusks, coral) are abundant and where waves continually wash away finer mud, leaving clean, coarse shell material to lithify.

Look for a rock made of visible, loosely cemented shell fragments — it looks like compacted seashells, which is essentially what it is.

It is soft (Mohs ~2-3), porous, lightweight, and fizzes in dilute acid like other carbonates. The texture is rough and gritty with obvious bioclasts.

Look-alikes: Other shelly limestones (more thoroughly cemented and denser), fossiliferous limestone (whole fossils in a fine matrix), and coral rock. Coquina's hallmark is the loose, abundant shell hash with high porosity.

Coquina has been an important regional building stone, especially in Florida and along subtropical coasts, used for forts, houses, and roads. Its porosity makes it a good insulator and, when fresh and slightly soft, easy to quarry and shape.

Crushed coquina is used as road base, aggregate, and in agriculture as a soil-sweetening lime source.

It is not used in jewelry due to its softness, but it is geologically valuable as a record of ancient coastlines and shell-rich ecosystems. Polished coquina slabs are sometimes used decoratively.