Netherrack

Netherrack is a block that can be found naturally generating in the Nether. Netherrack is the only block that Crimson Saplings can be placed on whilst remaining stable (Blackstone allows them to be placed, but they pop off with a block update). Any fire made on Netherrack does not naturally extinguish, but fire can only be made on the top.

Breaking
Netherrack can be mined with any pickaxe.

World generation
Netherrack can be found naturally generating in the Nether.

Miscellaneous
Netherrack is required for conveniently growing Crimson Saplings.

Netherrack allows fire to remain atop it forever, thus allowing builds to utilize fire.