Diamond Armor is given to you when you first join and can be bought from the Marcus NPC.
OBTAINING
Diamond Armor is first obtained in your Starter Kit, given to you when you first join and can also be given for free from Marcus.
ABILITIES
Diamond Armor provides 2.5% damage reduction per Protection level, in addition to the vanilla reduction effect.
UPGRADING
Diamond Armor can have its Protection level upgraded through the Marcus NPC, for an increasing Coin price each level. The only exception to this is the Counter-Offensive upgrade applied to Diamond Chestplates at Protection III, which also costs Moon Stones.
Diamond Armor can also have Runes applied to it. This is done through the Runecrafter NPC.