Netherrack

Hellstone (aka Bloodstone, Netherstone, Red Mossy Cobblestone, Brimstone) is a stone that is found only in the The Nether. It burns indefinitely when lit on fire, so it is a good way to have everlasting light. If Hellstone is set on fire, the fire will not spread to flammable blocks placed above the flame. Hellstone also has an extremely low resistance to TNT, and can be mined with as little as a stone pick with one hit. With hand or other, can be broken fairly easily but no material is dropped. The dark red of the stone, along with its abundance in The Nether give it a very chilling ambiance, and also make the region seem darker than it is. Hellstone also give off a small amount of light estimated to be the same as the nightime lumination from the moonlight. It is never completely dark in The Nether, even on blocks far away from light sources.

It currently has only one use as of Alpha version 1.2.0_02, which is the ability to burn forever, once lit on fire using Flint and Steel. It lacks any kind of recipe, though.

Hellstone and imported Earth blocks can be destroyed by the Ghast's fireballs. Cobblestone/Stone cannot. Other materials that cannot be destroyed are diamond, Iron Doors, and Obsidian. Stone buttons can be destroyed, though. It's unknown whether any other materials can be destroyed in the same way.