Chiseled Netherrack is a new decorative block added with NetherX. It is crafted by putting

Chiseled netherack in a nether dungeon.

four netherrack in a square on a crafting grid. It also makes up the walls of the nether dungeons if you want to find it. It can be crafted into slabs and stairs for more decoration. It is not entirely made for decoration, for it has a much higher blast resistance than netherrack. You can also find charred chiseled netherrack (the brownish ones) in the nether dungeons.