Final answer:
Coral reefs are made from the calcium carbonate skeletons of coral organisms that belong to the phylum Cnidaria, specifically using a form of calcium carbonate called aragonite. Their skeletons act as a vital habitat for marine life and are strengthened by materials such as magnesium.
Step-by-step explanation:
The rock-hard coral skeleton is composed of calcium carbonate (limestone) and typically a form of calcium carbonate known as aragonite. Coral reefs are built from the skeletons of tiny marine animals called corals, which belong to the phylum Cnidaria. These organisms are sessile polyps that can catch and feed on small creatures using their tentacles. Over time, their limestone skeletons accumulate, building the large, intricate structures we recognize as coral reefs. Scleractinian corals, which may form as solitary individuals or as colonies, always have skeletons made of aragonite, a crystalline form of calcium carbonate. This material provides the corals with a sturdy framework and is crucial for the vast diversity of life relying on the reef ecosystem. The presence of magnesium in some coral structures can enhance their strength significantly, as seen in the reinforced magnesium-rich tips of sea urchin teeth, which are also made of calcite, a form of calcium carbonate.