Coquina is a distinctive variety of fossiliferous limestone. Coquina is composed of finely-busted up seashells, typically clam shells & snail shells (bivalves & gastropods). Coquinas have high porosity, that is a significant amount of empty space between the grains. This results in coquina having a "spongy" appearance. As are all limestones, coquinas are composed of calcite (CaCO3) and they bubble in acid.
Coquina had military significance in early American history. Some military forts in Florida were constructed with coquina walls. The coquina "sucked up" cannonballs that were shot at fort. Coquina is a fairly common sedimentary rock in the Pleistocene of southeastern America (e.g., Florida).
Coquina from the Anastasia Formation (Upper Pleistocene to lower Holocene) of Florida, USA.