This is the most powerful known artifact in Amber whose existence is proven. Close to Amber it is hard to shift shadow and mold it, but with the Jewel, you could turn Kolvir into a giant potato or turn the Gulf of Amber to blood. It can also control weather with ease, and be used in some way to store spells and do other tricks Gendo never really explained to anyone. Derith is rumored to have used it to seal way the Flawed Amber known as Embra.
King Tylor wears it on court occassions, and the rest of the time, it remains locked in the royal vault. The Jewel of Judgement of Arcadia is held 24/7 by the Bride of the Jewel, who is likely to blast you to bits if you even look at it funny.
Five elemental reflections of it are known to exist, the Demi-Jewels of Judgement.