Coral

Coral is a solid block which comes in 5 colored variants: blue, pink, purple, red, and yellow. There also is a sixth dead variant.

Coral plants are blocks that can only be placed underwater. Like coral, they come in 5 variants, but they do not have a dead variant.

Obtaining
Coral will only drop as an item if mined with any type of pickaxe. Otherwise, it will drop nothing.

Coral and coral plants are not obtainable in Survival mode, and can only be accessed with the Creative inventory or using commands.

Usage
Coral and coral plants can only be used as a building 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 dead coral.

Trivia

 * Coral was also jokingly teased in the fake snapshot 12marc40awesome, along with fish mobs and "fish blocks".