Thin layers of calcium carbonate. Thin layers of calcium carbonate.
Yes. Reef-building corals are made up of two members, an animal and a plant that lives inside of the animal. The animal portion is a relatively simple creature called a polyp, a miniature sea anemone that resem­bles a flower, with a circle of tenta­cles surrounding a mouth. Each polyp is like a tiny anemone (or jellyfish on its back) which secretes a calcium carbonate, or limestone skeleton on/in which it lives......that means corals are made up of calcium carbonate(which bones are made up of) which are secreted by the polyps. For protection and support, hard corals build skeletons made of calcium carbonate. To do this, a coral polyp secretes layer upon layer of calcium carbonate underneath its body. As time goes by, the skeleton grows larger and larger, and the polyp lives on its outside edge.  Coral reefs are diverse underwater ecosystems held together by calcium carbonate structures secreted by corals. Coral reefs are built by colonies of tiny animals found in marine waters that contain few nutrients. Most coral reefs are built from stony corals, which in turn consist of polyps that cluster in groups. The polyps belong to a group of animals known as Cnidaria, which also includes sea anemones and jellyfish. Unlike sea anemones, corals secrete hard carbonate exoskeletons which support and protect the coral polyp However, unlike rocks, corals are alive. And unlike plants, corals do not make their own food. Corals are in fact animals. The branch or mound that we often call “a coral” is actually made up of thousands of tiny animals called polyps. A coral polyp is an invertebrate that can be no bigger than a pinhead to up to a foot in diameter. Each polyp has a saclike body and a mouth that is encircled by stinging tentacle Coral reefs are built by and made up of thousands of tiny animals—coral “polyps”—that are related to anemones and jellyfish. Polyps can live individually (like many mushroom corals do) or in large colonies that comprise an entire reef structure.  Reefs provide homes for more than 4,000 species of fish, 700 species of coral, and thousands of other plants and animals. The architects of coral reefs are hard corals. Unlike soft corals, hard corals have stony skeletons made out of limestone that is produced by coral polyps. When polyps die, their skeletons are left behind and used as foundations for new polyps.  A coral head is a colony of myriad genetically identical polyps. Each polyp is a sac-like animal typically only a few millimeters in diameter and a few centimeters in length. A set of tentacles surround a central mouth opening. An exoskeleton is excreted near the base. These are commonly known as zooxanthellae and the corals that contain them are zooxanthellate corals. Such corals require sunlight and grow in clear, shallow water, typically at depths shallower than 60 metres (200 ft). Coral Skeleton Like most animals, coral have a skeleton, but unlike mammals and fish, their skeleton is formed by the outer skin and is ex­ternal to the polyp. This exoskeleton is made up of a white, chalky material called calcium carbonate, which is known more commonly as limestone. Much of the limestone that makes up north­ern Saipan is made up of the remains of corals and other animals and plants that form calcium carbonate skeletons. Each polyp is like a tiny anemone (or jellyfish on its back) which secretes a calcium carbonate, or limestone skeleton on/in which it lives......that means corals are made up of calcium carbonate(which bones are made up of) which are secreted by the polyps. A coral reef is made of thin layers of calcium carbonate. Coral polyps form a living mat over a calcium carbonate skeleton. 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. Coral reefs are built by coral polyps as they secrete layers of calcium carbonate beneath their bodies. The corals that build reefs are known as “hard” or “reef-building” corals. Soft corals, such as sea fans and sea whips, do not produce reefs; they are flexible organisms that sometimes resemble plants or trees. 
https://answers.yahoo.com/question/index?qid=20061102013044AABSZCh http://www.teachoceanscience.net/teaching_resources/education_modules/coral_reefs_and_climate_change/what_is_a_coral/ https://en.wikipedia.org/wiki/Coral_reef http://floridakeys.noaa.gov/corals/coralanimals.html http://coral.org/coral-reefs-101/coral-reef-ecology/coral-polyps/ http://floridakeys.noaa.gov/corals/coralreef.html https://en.wikipedia.org/wiki/Coral https://answers.yahoo.com/question/index?qid=20061102013044AABSZCh http://oceanservice.noaa.gov/facts/coralmadeof.html http://coral.org/coral-reefs-101/coral-reef-ecology/how-reefs-are-made/
what are corals made of