Nether Wart

Nether wart (also known as nether stalk) is a plant grown in The Nether. It can be found within nether fortresses on soul sand. It currently does not spawn on any other blocks and will not respawn when taken from fortresses. Nether wart can be placed onto soul sand by right-clicking (as if it were a seed), where it will grow.

Growth
Nether wart can only be planted on soul sand. It can be planted in any dimension, but it will only grow in The Nether. In the Overworld or The End, it will not grow, but does remain planted.

When planted in the Nether, a nether wart will grow through four distinct stages, though the middle two stages have the same appearance (this can be verified by using a BUD switch to detect the stage transitions). Growth is driven by block ticks, similarly to wheat. The duration of each stage is random, with no upper or lower limit, but the average duration is about 3 minutes and 30 seconds. The growth rate is not affected by light or any other environmental factors. Bone meal has no effect on the nether wart.

Because nether wart grows rather slowly, large farms are recommended to maintain an abundant supply. A great deal of time must be spent in the Nether to give the nether wart time to grow. A player might pass this time brewing potions from previously harvested warts, or killing some blazes for example.

Harvesting
Nether warts are ready to harvest when they reach their third stage (when the warts appears dark crimson and there is no space between the blisters). Breaking a full grown nether wart yields 2 to 5 nether wart seeds. Breaking it before it's fully grown yields only a single seed.

When harvesting a large field of nether warts, some players find it beneficial to construct a viewing bunker one block lower than the growing nether wart. When standing on the viewing platform, the player's vision is lined up with the nether wart in such a way that all the unfinished nether wart create a single horizontal line with the tops of them. Any protrusion from this horizontal line signifies that there is a matured nether wart plant ready for harvest.

Trivia

 * Nether wart will not break when stepped on, nor will Ghast fireballs destroy them (though fire caused by the fireballs will).
 * It makes the same sound as stone when destroyed.
 * In the code, nether wart is called "netherStalk" for the block and "netherStalkSeeds" for the item.