The Orichalcos Stone is the crystal that powers "The Seal of Orichalcos" in the Waking the Dragons arc, named for the legendary material "Orichalcum" featured in one of Plato's dialogues which also contained the characters "Critias" and "Timaeus". They were created by The Great Leviathan.
To wield a fragment of the stone, the owner must be deemed worthy by it. If so, they are able to hold the stone in their hand and be granted great power. If they're not, their souls are taken by the stone and fed to the Great Leviathan. The stones are maintained by the darkness within the owner's heart, just like the Seal of Orichalcos magic card. The darker the owner's heart, the more power he can access. But, if they overcome the darkness, the stone will shatter to pieces.
While under the influence of the Orichalcos, their minds are warped with psychological disruption which causes them to lose their personal self to the darkest abyss. It may also change the way people view other things differently such as when Mai changed her outfit from white and purple to black leather and purple while she was possessed. While Yami Yugi activated the Seal of Orichalcos, his personality became heartless and cruel which explained the reasons his revived monsters felt betrayed; but, Yugi's changed personality while playing the same card that Yami used against Rafael was derisive and cruel which also described as a cunning sociopath (which basically opposite of Yami's reflection) claiming Yami's defense strategy was useless since Yugi's monsters gets a power bonus due to the Seal's effect; which wanted Yami to feel the same pain they suffered at the hands of his betrayal. Yami tried to reason with Yugi not to make the same mistake he did in his recent actions; but Yugi considered his reason as a sign of weakness which show no remorse for him and continue to attack him as a possible mark of revenge (but, this is the only way to emancipate Yami's heart from the darkest abyss).
While playing the Seal of Orichalcos with or without the stone, their Dueling style and strategy becomes brutal and violent; that they can lose reason to use their own will to Duel like Yami Yugi did when he Dueled against Rafael; he begins to disrespect his cards and sacrifice his own monsters using "Catapult Turtle" in order to win (which in fact both Yugi and Yami Yugi didn't allow to use it for the entire Duel in the past like the Battle City Tournament and Noah's Virtual World, because they only rely on a true new strategy instead of using sacrificing monster for a direct attack on his opponents to win). Yugi also uses this strategy in order for Yami Yugi to counter this monster sacrifice which can release him from the Orichalcos' malignant influence which in fact corrupted him and the darkness that dwells in his heart.
The people who are chosen by the stone acquire strength and mystical powers that rival the Millennium Items. They have the power to materialize Duel Monsters cards. When inside the seal, it even allows the owner to control an Egyptian God. When the stone is thrown to the ground, it summons one or more Orichalcos Soldiers. A wielder of a stone can also destroy the soldiers at will, like Valon did to spare Joey Wheeler. They also have the power to unlock and dispel "The Seal of Orichalcos", but only from the outside, and at the risk of the owner's soul. Other abilities include casting illusions, teleportation, and showing mental visions.