Ah yeah, that makes sense actually. Thanks!
Edit: actually, this sounds like a good idea, but runs into some problems.
Firstly, it doesn't seem to be working (though I'll do some more troubleshooting)
and secondly, wouldn't this also replace the varieties in GT's different granite types too? If I only replace the stone varieties of the ores, then I'll have enough for normal gameplay, and the special stone types can stay, making the veins that generated in say red granite still look right. I suppose I could do a PlacesBeside check but that feels clunkier and I feel as though it would add to the generation time. Perhaps I'm asking for too much there though if it's not possible to replace by NBT; oh well!