What is the main mineral compound that makes up coral skeletons?

The main mineral compound that makes up the skeletons of hard corals is calcium carbonate. Coral polyps secrete this mineral to build the rigid framework that supports and protects their colonies.

In stony corals, calcium carbonate is deposited around cup-like chambers called corallites, where individual polyps live. The mineral commonly occurs in the crystal forms aragonite or calcite, combined with a smaller organic framework of proteins, sugars, and lipids. Over generations, the accumulated skeletons of countless colonies become the physical foundation of coral reefs.

Coral is an animal, not a plant or a rock. Many reef-building corals also host photosynthetic dinoflagellate algae, traditionally called zooxanthellae, which provide energy to the coral but do not make up its skeleton. Soft corals differ because they generally lack the massive external skeleton characteristic of hard corals.

Ocean acidification can make skeleton-building more difficult by reducing the availability of carbonate ions, potentially producing weaker, slower-growing reef structures.