Mar-a-Lago is built from coquina, a porous limestone that absorbs cannonball impacts and hurricane force without cracking.

Coquina is a form of limestone composed almost entirely of shell fragments from mollusks, trilobites, and corals. The coquina used at Mar-a-Lago was quarried locally in Florida, where the rock formed over thousands of years as calcium carbonate cemented these fragments together. Because the rock is highly porous, it has a unique mechanical property: it compresses upon impact rather than fracturing like granite or marble.The most famous historical demonstration of this durability occurred at the Castillo de San Marcos in St. Augustine, Florida. During the 1702 siege by British forces, soldiers were astonished to see their cannonballs bury themselves in the fort's walls without causing structural failure. This "sponge effect" works because air pockets between the shell fragments allow impact energy to dissipate locally rather than concentrate at a single point.Mar-a-Lago was constructed between 1924 and 1927 for cereal heiress Marjorie Merriweather Post. Architect Marion Sims Wyeth selected coquina for both its aesthetic beauty and its proven resilience against Florida's tropical storms. Modern geological studies confirm that coquina actually strengthens over time through a process called case hardening.As the rock is exposed to salt air and moisture, calcium carbonate partially dissolves and then re-precipitates on the surface. This creates a harder exterior shell while maintaining the flexible, shock-absorbing core. This combination of geological properties makes the estate one of the most structurally sound historic buildings in the United States.