Coral

Coral were sedentary invertebrates of the class Anthozoa, and phylum Cnidaria. They secreted a calcareous skeleton. It was notable that they were, in fact, animals, despite their plant-like appearance. (AUDIO: Chase the Night)

Coral on Earth
Mermen used coral to make their homes and weapons. (AUDIO: Cryptobiosis)

On 21 October 1829, the Mermaid hit a coral reef in a storm. The ship was wrecked but none of the crew perished. (PROSE: Of the Mermaid and Jupiter)

The First Doctor once fished the coral reefs off the Santa Cruz Islands. (PROSE: The Devil Goblins from Neptune)

Tegan Jovanka once almost drowned while swimming near a coral reef. (PROSE: The Sands of Time)

Ultimately, some humans themselves evolved to be partially made out of coral. Two posthumans at the First Auction in Heaven, the Baron of Trilermeriaq and his lover, were mostly made of yellow coral, with "swaying sacks of brain". (PROSE: Going Once, Going Twice)

Coral on other planets
Reef, the capital city of Hydron, was made almost completely of coral. (PROSE: The Underwater War)

Xanadu, a planet which had been designed by Welwyn Borr, had beautiful coral reefs, some of which were poisonous. (PROSE: The Tomorrow Windows)

Orbis had coral, some of which encrusted the control device of Morbius' stellar manipulator after resting on the ocean floor for six centuries. (AUDIO: Orbis)

Polypia 4 had glowing coral which was actually a giant brain. (COMIC: Coral Maze)

Kataa Flo Ko had diamond coral reefs. (TV: The Sontaran Stratagem)

Halcyon had trees which resembled coral. (AUDIO: The Resurrection of Mars)

On 16 Alpha Leonis One, the Cthalctose lived in buildings shaped like coral reefs. (PROSE: Eternity Weeps)

Coral was not found exclusively on planets. Space coral reefs was a hazard for spacecraft. Commander Gammades's ship and Lady Vuyoki's ship both became marooned on one such reef in a bubble outside of time. (AUDIO: Time Reef)

The Doctor's TARDIS had one control room which appeared to be made of coral or a coral-like substance. (TV: Rose et al.)