Hard corals grow in colonies and are the architects of coral reefs. Hard corals—including such species as brain coral and elkhorn coral—create skeletons out of calcium carbonate (also known as limestone), a hard substance that eventually becomes rock. Hard corals are hermatypes, or reef-building corals, and need tiny algae called zooxanthellae (pronounced zo-zan-THEL-ee) to survive. Generally, when we talk about coral, we are referring to hard corals.
The variety of coral shapes and sizes largely depends on the species. Some corals form hard and pointed shapes, while others form soft and rounded shapes. The shape of coral colonies also depends on the location of the coral. For example, in areas with strong waves corals tend to grow into robust mounds or flattened shapes. In more sheltered areas, the same species may grow into more intricate shapes with delicate branching patterns.