Coral

Coral was a substance created by marine life forms. It sometimes formed into reefs which were hazardous to ocean-going vessels.

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)

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.)