Diamondsteel

You touch a suit of metal armor, giving it incredibly resilience. For the duration, whenever the creature wearing the armor would take Bludgeoning, Piercing, or Slashing damage from a Non-Adamantine weapon, reduce the damage by 4.   At Higher Levels. When you cast this spell using a higher level spell slot, increase the damage reduction by 1 for each slot level above its base level.
 

Transmutation Level: Paladin 3, Sorcerer 3, Wizard 3, Components: V, S, M, Casting Time: 1 standard action Range: Touch Target: Suit of metal armor touched Duration: 1 round/level Saving Throw: Will negates (object) Spell Resistance: Yes (object)   _You pass your hand over the suit of armor several times before finally touching it. As you do so, you feel a warmth grow in the palm of your hand. The warmth passes into the armor and manifests as a sparkling shine._   Diamondsteel enhances the strength of one suit of metal armor. The armor provides damage reduction equal to half the AC bonus of the armor. This damage reduction can be overcome only by adamantine weapons. For example, a suit of full plate would provide damage reduction 4/adamantine, and a +1 breastplate (+6 AC) would provide damage reduction 3/adamantine.   Material Component: Diamond dust worth at least 50 gp.