Re: Ask the RPoLians; crowdsourceing answers for your game!
kouk: Granulite. It's a different type of rock. The market for the stuff is apparently really small compared to other materials like marble and granite. This is the strongest correlation I have found between the two: "The production of granite by metamorphic heat is difficult, but is observed to occur in certain amphibolite and granulite terrains."
Godzfirefly: Deep continental crust. You'd pretty much have to dig for it. You would absolutely have to dig for it if you include metaphorical forms of digging (which is, you know, why I ask).
AussieKnight87: The size is not extremely important, and, actually, the end result is supposed to be octahedral. Slate, which is commonly referred to as bluestone in southern Australia (and I think it's safe to assume you're from there), is not a granulite. Granulite is medium-to-coarse grained, while slate is actually the finest-grained of all metamorphic rock, just to give an example.