What mineral form makes up most stony coral skeletons?

What mineral form makes up most stony coral skeletons? The answer is aragonite.

Aragonite is the crystalline form of calcium carbonate that makes up the skeletons of most modern stony, or hard, corals. Individual coral polyps secrete this mineral at their bases, building cup-like structures called corallites. As colonies grow and new polyps bud, these small deposits accumulate into the rigid frameworks associated with coral reefs.

Aragonite and calcite have the same chemical formula, CaCO₃, but their crystal structures differ. Aragonite is generally less stable than calcite under ordinary surface conditions and can gradually transform into calcite over geological time. This difference helps explain why ancient coral skeletons are often poorly preserved compared with their original form.

The answer is not universal for every coral in every era. Modern scleractinian corals primarily produce aragonite, while some extinct coral groups formed calcitic skeletons. Soft corals generally lack a large solid skeleton, although they may contain tiny calcium-carbonate elements called sclerites. Ocean acidification is a concern because lower carbonate-ion availability can make aragonite formation more difficult and weaken reef growth.