Define renewable. In a world of 29999999 blocks (did i add enough nines?), squared; every resource is renewable, if you travel far enough. I challenge someone to use all the coal mineable in a world. Or even to gather all the diamonds. The only non-renewable resource is the dragon egg, and you can get more of those if you farm enough mystcraft ages. Thus making the only non-renewable resource: Time.
So what if we'll never use all the Coal in a world? There's still a finite amount of it, and when it runs out, it runs out. The point at which it runs out is ridiculously late, but that doesn't mean it's nonexistent.
Okay. Perhaps I should make myself clear about my definition of renewable. A resource is renewable if and only if, for any positive number n, it is possible to obtain n of the given resource, using any method necessary or none at all to store the resources. I don't mean any
reasonable number, if I asked you to produce 2000000000 coal, would it be possible? Probably, though it would be ridiculously time-consuming. If I asked you to produce 2000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000000 coal, would it be possible? Not a chance. (Note that we're considering an infinite amount of RAM allocated with a save file of infinite size, with a computer that takes 0 time to perform any calculation, since I like dealing with ideals.)
You do make a good point about Mystcraft ages. Is the age stored as a Java int? In which case the number of ages is (incredibly huge but) finite, thus not allowing us to obtain more Coal once we run out.
Though, I do think Coal, along with a heck of a lot of other resources, is renewable via Scrapbox. The point is, a resource being obtainable through exploration doesn't make it renewable.