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The Adamant Orb provides Dialga a boost for its Dragon and Steel attacks, without the drawbacks that Life Orb and Choice Specs have. With the focus of many of Dialga's sets being on attacking with its powerful STAB Dragon moves, this item would seem like an obvious choice. It is indeed quite effective, especially when combined with stat-boosting moves such as Bulk Up.
The downside to the Adamant Orb is the fact that it only benefits Dialga's Dragon and Steel attacks, as opposed to all of its possible attacks. Furthermore, Dialga never uses any Steel-type attacks in the Uber metagame, because Dialga has a very versatile movepool, and can usually hit opposing threats harder with one of its other moves. The Adamant Orb boosts do not apply to these attacks, so running a Life Orb or Choice Specs is a better option on most sets. Nonetheless, the Adamant Orb is noteworthy for being a non-recoil item that provides a significant boost to Dialga's most used attacks.