Adamantine

Adamantine Armor
Adamantine armor provides damage reduction based on the type of armor that is made from it:
 * Light: 1/- damage reduction
 * Medium: 2/- damage reduction
 * Heavy: 3/- damage reduction

This damage reduction does not stack with that granted by other abilities, such as the Barbarian's damage reduction. (Note: this is per the PnP game. Need confirmation if this is true in DDO)

Adamantine Weapons
Adamantine weapons may bypass the damage reduction of certain items and creatures. They are especially useful against golems.