That's it. That's the answer.
It's a composite stone made of compressed seashells with carbonic acid leaching out calcium out of the shells to fuse them together into stone.
The above video, from the US National Park Service, explains the material of the walls of Castillo de San Marcos and how its porosity was both a weakness (it needs to be plastered to waterproof it) and a strength (the walls could 'absorb' cannon balls fired at it in an attack.)
The second video - below - explores the preservation efforts necessary to maintain the coquina walls.
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