Given the adamantine quote, the rules seem to assume that a quarterstaff is uncapped. However, since the rules do not preclude it, I'd rule that they can be capped, and a capped quarterstaff would then qualify to be made adamantine, cold iron, or silvered, but not mithral (which requires most of the weapon be metal, rather than merely that the weapon have metal parts.)
This may be a House Rule, though, since the adamantine rule does seem to indicate the designers assumed a quarterstaff to be naked wood.