Yes, it's the red 'gravel' in the Nether, most of the scenery is made out of it.
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Yes. You can find Nerherrack burning naturally no problem in the Nether. There's a ton of buring netherrack there. Minecraft is Awesome :D
Yes. Unless you put it out.
Yes, in the Nether Zombie Pigmen spawn on the netherrack but I don't think they spawn on Netherrack in the Overworld.
As far as I'm concerned, you cannot make a pokeball. I have been playing minecraft ever since it came out, but maybe you could make a non-usable one our of Netherrack (underworld "dirt") and blocks of iron. For the tiny clasp, you could use obsidian.
The only way to keep a fire going indefinitely is to get a block of netherrack, if these are lit on fire, the block is neither consumed, nor extinguishes.