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From Terraria Wiki
Coral can be found in the Oceans at the ends of the world. It grows on Sand Blocks within 250 tiles of the edges of the map and above the depth of 0 feet. The sand must be under at least 5 tiles of water. Background walls block its growth.
It can be harvested either with a pickaxe, by mining out the sand underneath it, or by placing a block next to it. It can be placed as a decoration and will have one of many appearances, much like Books. While each piece of coral only needs one block under it, it essentially takes up an extra half-space on each side, preventing walls or objects from being placed directly to either side. For example, to place 5 coral between walls, you would need the walls to be at least 11 blocks apart. To fit the same 5 coral on an open platform, it only needs to be 9 blocks wide.
- Coral are actually animals, not plants. However, there are some species of coral that have algae living inside the coral polyps making them both plants and animals.
- 1.2.4: Now used for crafting the Sonar Potion.
- 1.2: Coral must be collected by a pickaxe, stack limit increased from 250 to 999.