Coral

Coral is an un-obtainable block that comes in 5 colored variants: blue, pink, purple, red, and yellow. There also is a sixth dead variant.

Obtaining
Coral is not obtainable in Survival mode, but can be found in the Creative inventory.

Usage
Colored coral must be in water in order to stay alive. If it's placed outside of water, it will instantly and irreversibly transform into dead coral.