Like other Unique items in Path of Exile, Unique Jewels are powerful and build-defining. They offer ways to diversify your areas of specialisation, or to double down on a strategy that is working well for your build.
Some Unique Jewels interact with passive skills in a radius around them - either changing their properties, triggering when certain thresholds are met or altering the rules of the tree itself.
A quest in Act Two offers you a different selection of Unique Jewels to choose from in each difficulty level.
Magic and rare Jewels can be crafted using the same set of Orbs that affect other items in Path of Exile. By rerolling and modifying your Jewels, you're able to essentially create your own Notable Passives, some of which can be substantially more powerful than those offered by the tree itself.