Coral is mostly made of the hard mineral calcium carbonate (a form of limestone) plus living soft tissue called polyps that sit on and build that skeleton over time.

What is coral made of? (Simple)

  • The skeleton of most reef-building corals is made of calcium carbonate (CaCO₃), usually in the crystal form aragonite or calcite.
  • The living part is a tiny animal called a polyp: a clear, tube-shaped body with a mouth surrounded by tentacles.
  • Together, millions of these polyps and their limestone skeletons form large coral colonies and, eventually, entire coral reefs.

A closer look at the structure

You can think of coral as a combo of “animal + rock”:

  1. Polyp (the animal part)
    • Made of soft tissues (cells, proteins, lipids, sugars) like other simple marine animals.
    • Has stinging tentacles to catch tiny prey and defend itself.
  1. Skeleton (the mineral part)
    • A biocomposite of minerals plus organic material: mostly calcium carbonate with small amounts of proteins, sulphated sugars and lipids that help control how the crystals grow.
 * Each polyp secretes a thin layer of CaCO₃ beneath itself, adding to the “rock” base over time.
  1. Hard vs soft corals
    • Hard (stony) corals : strong calcium carbonate skeletons; they are the main reef builders.
 * **Soft corals** (like sea fans): have less rigid internal structures and do not build big limestone reefs, even though they are still made of animal tissue with some supporting elements.

How reefs are built from what coral is made of

  • Reef-building corals pull calcium and carbonate ions from seawater and turn them into solid calcium carbonate (a process called biomineralization).
  • As polyps grow and die, new ones build on top of old skeletons, stacking thin layers of limestone for centuries to millennia.
  • Over time, this creates massive reef structures that can weigh many tons and host a huge variety of marine life.

Mini FAQ and extra angles

  • Is coral a rock, a plant, or an animal?
    It is an animal that builds a rock-like calcium carbonate skeleton, and many species also host symbiotic algae in their tissues.
  • Why does coral look so colorful if the skeleton is just limestone?
    The bare skeleton is pale; the colors mostly come from the living tissue and symbiotic algae living in and on the polyp.
  • What is a coral reef, then?
    A coral reef is a thick, layered mass of calcium carbonate skeletons from many generations of corals, still covered on the surface by living polyps.

TL;DR:
Coral is made of tiny animals (polyps) whose bodies are soft tissue but which constantly secrete a hard calcium carbonate skeleton; stacked over time, those mineral skeletons plus the living polyps form coral reefs.

Information gathered from public forums or data available on the internet and portrayed here.