Coral reefs at Johnston Atoll in the North Pacific.
Stony corals (or scleractinians) are the corals primarily responsible for laying the foundations of, and building up, reef structures. Massive reef structures are formed when each individual stony coral organism—or polyp—secretes a skeleton of calcium carbonate.
Most stony corals have very small polyps, averaging one to three millimeters (0.04 to 0.12 inches) in diameter, but entire colonies can grow very large and weigh several tons. These colonies consist of millions of polyps that grow on top of the limestone remains of former colonies, eventually forming massive reefs.
In general, massive corals tend to grow slowly, increasing in size from 0.5 to two centimeters (0.2 to 0.8 inches) per year. However, under favorable conditions (lots of light, consistent temperature, moderate wave action), some species can grow as much as 4.5 centimeters (1.8 inches) per year.
For more information:
NOAA Coral Reef Conservation Program
U.S. Coral Reef Task Force
NOAA Coral Reef Information System
Coral Health and Monitoring Program
Five Things You Should Know About Coral Reefs
Value of Coral Reefs (audio podcast)
Coral Reef Ecosystems (NOAA State of the Coast)