It's in one of the Man Of Steel concept/making of/art of books.
I forget which. My Brother got it for his birthday and i got a read afterwards.
It explains that for millennia, the Kryptonians have been mining all the minerals from the core of Krypton, digging ever deeper, stripping away the mineral wealth(By the time they decided to stay on Krypton after the great expansion, almost all metals were gone). Also how they had to make do with what they had lying around, so their technology has harnessed repurposing any and all matter at their disposal and that included animal bones.
I will find out which book it is.