As a swift action, the wielder of this weapon may trasmute the alloy to a different material. An iron sword can become Cold Iron, Adamantine, Alchemical Silver or Mithral.
I thought this wopld be cool. Essentially, rather than carrying around three weapons for your damage reduction overcoming needs, you could just add this ability to your weapons and trasmute it as the situation calls for. The only real problem I see with this is you can save a couple grand on enchants for Cold Iron weapons, but not a game breaking amount.