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Why do eggshells and many marine animal shells contain calcium carbonate?

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A labeled diagram of the periodic table entry for calcium, showing its atomic number 20, symbol Ca, atomic weight 40.078, electron configuration, and classification as an alkaline-earth metal. Icons indicate calcium is a solid with a face-centered cubic crystal structure and strongly basic oxides at room temperature. — calcium The chemical element calcium is the most abundant metallic element in the human body.

Eggshells and the shells of many marine animals, such as mollusks and corals, contain calcium carbonate because it serves as a primary structural material for their protective skeletons and coverings.12

These organisms utilize calcium carbonate to build stiff, durable structures that provide essential protection for their soft tissues. In marine environments, these calcareous structures are often secreted by the animal's tissues—such as the ectoderm in corals or the mesoderm in echinoderms—to form rigid shells, ridges, or ossicles. Because calcium carbonate is stable and can be deposited in layered or branched forms, it allows these animals to create robust defenses that can withstand environmental pressures.12

While calcite is the most common stable form of this compound, some marine organisms also incorporate magnesian calcites into their skeletal material. Ultimately, the use of calcium carbonate is a widespread biological strategy for creating the hard, protective exteriors necessary for survival.231