Coral

Coral is a solid block which comes in five colored variants: blue, pink, purple, red, and yellow. Every color has a different texture.

Dead coral is a solid block which comes in five different variants: blue, pink, purple, red and yellow.

Coral plants are blocks that can only be placed underwater. Like coral, they come in five variants, but they do not have a dead variant.

Coral fans are blocks that can only be placed underwater, on the 4 sides of blocks: north, south, west, east. Like coral, they come in five variants, but they do not have a dead variant.

Obtaining
Coral and dead coral will only drop as an item if mined with any type of pickaxe. Otherwise, they will drop nothing.

Coral plants and fans can be instantly mined and will always drop themselves.

Natural generation
Coral and coral plants naturally generate in coral reef structures. These are found in warm oceans.

Usage
Coral, dead coral, coral plants and coral fans can be used as a building or decoration block.

Dead coral
In order for coral to stay alive, at least 1 of the blocks touching it must be water. If it's placed outside of water, it will instantly and irreversibly transform into its respective dead coral.

Trivia

 * Despite being called plants in the game, in real life corals are actually colonies of sessile (immobile) animals.