Rock

Coquina

Koquina

A porous limestone of squeezed-together sea shells: in Florida, cannonballs sank into a fort's walls instead of smashing them.

Also known as: Shell limestone

Popularly: Natural stone (building and decor)
Building stone

Photo: Wilson44691 · Public domain · Wikimedia Commons

In short

Coquina is a limestone made almost entirely of broken sea shells, with pieces big enough to see with the naked eye. It piles up on beaches and in shallow seas where waves grind and sort the shells. Because it is porous and soft it is easy to cut, and it has been used as a building stone in Florida for more than 400 years. Its name comes from a Spanish word meaning “cockle” and “shellfish”.

Photos

Coquina specimen (2) — Photo: Brett Cannon · CC BY-SA 4.0 · Wikimedia Commons

How does it form?

Coquina forms where waves and currents are strong, on beaches, submarine sand banks, tidal channels and coastal bars. Shells there are constantly rolled, broken and sorted by size: light fragments are carried away, while the heavier shell pieces stay and pile up. As new sediment buries them they are compressed, and calcite precipitated from water glues the pieces loosely together.

Where cementation is weak the rock stays crumbly; where it is good the result is a sturdier limestone (sometimes called coquinite). Coquina is found in rocks of every age from the Devonian to the present; the Anastasia Formation in Florida is Late Pleistocene, with its oldest samples about 110,000 years old.

What is it used for?

Building stone: In Florida coquina has been used for walls and buildings for more than 400 years. The Castillo de San Marcos fort is the best-known example; the early structures were protected with lime plaster.
Roads and fill: Crushed coquina is used on unpaved roads and as fill in highway construction.
Fertiliser and decoration: Because it contains phosphate it has been used as fertiliser, and it is also used as landscaping stone.
Water and oil: Being porous it can store groundwater, and coquina layers are reported to be reservoir rock in some oil fields off Brazil.

The stone that protected a fort

When the Spanish began building the Castillo de San Marcos at St. Augustine, Florida, in 1672, they brought the stone across the bay from Anastasia Island. Fresh from the quarry it was very soft, and it hardened as it dried until it could be used. In the sieges of 1702 and 1740, cannonballs are said to have buried themselves in the walls without breaking the stone. The fort is now a US national monument.

What determines its value?

Coquina is a cheap local raw material where it occurs, used for road fill, fertiliser and garden decoration. As a building stone its worth depends on how well the shells are cemented, the size and soundness of the blocks, how porous and water-sensitive it is, and how close the quarry is. In some places quarrying is banned to protect the landscape; in the Shark Bay World Heritage Site, for example, no more coquina can be taken.

This information is educational, not buying, selling or investment advice. Value varies enormously with quality and treatment.

How to recognise it

1
It is made of visible sea shells and shell fragments, usually a few millimetres across and packed tightly together.
2
It is very porous and rough on the surface, light for its size, and soaks up water at once.
3
Being calcium carbonate it fizzes when dilute acid or vinegar is dropped on it, and a steel knife scratches it easily.
4
The shell pieces often lie parallel to the bedding, and a fresh break shows cross-sections of shells.

Fakes and imitations

What shops usually don’t tell you:

Tabby is not coquina: it is a man-made concrete of burnt oyster-shell lime mixed with sand, ash and broken shells and poured into wooden forms. Coquina is a natural rock.
Limestones in which shell pieces float scattered in a fine mud or lime matrix are called coquinoid limestone, not coquina; in coquina the shells touch each other.
On the Atlantic coast of the USA the name “coquina” is also given to small Donax clams. These are living animals, not rock.

Myths and misconceptions

MythCoquina is a kind of concrete made with shells.
FactCoquina is a natural rock: shells piled up on a shore in past geological ages and became cemented together. The man-made shell-and-lime material is called tabby.
MythA fort built of soft stone is a weak fort.
FactThe coquina walls of the Castillo de San Marcos in Florida held out in sieges because cannonballs sank into the stone instead of shattering it. Hard stone can crack and burst on impact, while soft, porous coquina absorbs the blow.

Did you know?

More than 90% of the famous Odesa Catacombs in Ukraine are voids left by coquina quarried in the 19th century to build the city; the network is said to reach up to 2,500 km in length.

At Shell Beach in Western Australia the shore is piled 7–10 metres deep with the shells of a single cockle species, and these shells have cemented into coquina.

Freshly quarried coquina is so soft that it has to be dried for one to three years before it can be used.

Where is it found?

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Castillo de San Marcos, St. Augustine, Florida, USA · Historic sourceThe Spanish began the fort in 1672 and finished its core in 1695, using coquina quarried at the King's Quarry on Anastasia Island. In the sieges of 1702 and 1740, cannonballs sank into the walls rather than shattering them.
Washington Oaks Gardens State Park, Palm Coast, Florida, USA · Known localityOutcrops of coquina from the Anastasia Formation, which runs along Florida's east coast, can be seen on the beach. The formation is Late Pleistocene quartz sand and calcite coquina.
Fort Fisher, North Carolina, USA · Known localityA natural coquina outcrop sits here; large-scale mining of the rock for highway construction began at Fort Fisher in the 1930s.
Shell Beach, Shark Bay, Western Australia · Major depositThe beach is covered to a depth of 7–10 metres in shells of the Fragum erugatum cockle, which thrived without predators in the very salty water. The shell deposits have hardened into coquina; stone was once quarried for buildings in Denham, but quarrying is no longer allowed in the Shark Bay World Heritage Site.
Odesa Catacombs, Ukraine · Historic sourceMore than 90% of the tunnels are voids left by the coquina (shell limestone) quarried in the 19th century to build the city.

Locations are approximate to protect sites. Red dots mark places in Türkiye. Open in the big map →

Frequently asked questions

What is coquina?

What is coquina used for?

Building stone: In Florida coquina has been used for walls and buildings for more than 400 years. The Castillo de San Marcos fort is the best-known example; the early structures were protected with lime plaster. Roads and fill: Crushed coquina is used on unpaved roads and as fill in highway construction. Fertiliser and decoration: Because it contains phosphate it has been used as fertiliser, and it is also used as landscaping stone. Water and oil: Being porous it can store groundwater, and coquina layers are reported to be reservoir rock in some oil fields off Brazil.

How hard is coquina?

It has a hardness of 2–3 on the Mohs scale. A fingernail cannot scratch it, but a steel knife can. Its specific gravity is 1.3–2 g/cm³.

Where is coquina found?

Its best-known localities: Shell Beach, Shark Bay, Western Australia; Castillo de San Marcos, St. Augustine, Florida, USA; Odesa Catacombs, Ukraine; Washington Oaks Gardens State Park, Palm Coast, Florida, USA; Fort Fisher, North Carolina, USA.

How can you tell if coquina is real?

It is made of visible sea shells and shell fragments, usually a few millimetres across and packed tightly together. It is very porous and rough on the surface, light for its size, and soaks up water at once. Being calcium carbonate it fizzes when dilute acid or vinegar is dropped on it, and a steel knife scratches it easily. Tabby is not coquina: it is a man-made concrete of burnt oyster-shell lime mixed with sand, ash and broken shells and poured into wooden forms. Coquina is a natural rock.

Is coquina valuable?

On the “street to diamond” value ladder it sits on step 2 of nine: Raw material by the tonne. Coquina is a cheap local raw material where it occurs, used for road fill, fertiliser and garden decoration. As a building stone its worth depends on how well the shells are cemented, the size and soundness of the blocks, how porous and water-sensitive it is, and how close the quarry is. In some places quarrying is banned to protect the landscape; in the Shark Bay World Heritage Site, for example, no more coquina can be taken.