The Adamant square shield is the third-best standard square shield. It requires 40 Defence to wield. It can be smithed at level 78 Smithing with two adamant bars. Doing so yields 125 Smithing experience.
Adamant square shields can be received as loot from silver chests in the Shade Catacombs. This item is also used in the completion of a clue as part of a Hard level Treasure Trail.