Coquina
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Coquina is an incompletely consolidated sedimentary rock, mainly composed of mineral calcite, often including some phosphate, in the form of seashells or coral. It is created in association with marine reefs. While not usually referred to as such, it is actually a subset of limestone.
Coquina is quarried or mined as a source of paving material. It is usually poorly cemented and easily breaks into component shell or coral fragments, which can be substituted for gravel or crushed harder rocks. Large pieces of coquina of unusual shape are sometimes used as landscape decoration.
Because coquina often includes a component of phosphate, it is sometimes mined for fertilizer.
Occasionally used as a building stone in Florida throughout its history, it formed the walls of Saint Augustine. One can still see some cannonballs embedded in the wall that sunk into the soft rock during bombardments.