The Jewel of Lorelei has the power to disperse Seventh Fonons. It's supposed to work in tandem with the Key when they're activated with a hyperresonance- both were part of the pact between Yulia Jue and Lorelei.

[He lifts his hand, studying it for a moment. The Jewel had left an odd, familiar tingling sensation on his fingertips, and he knows he'd felt it before. The last time Luke had given him the Jewel, in fact... when they'd tried to hyperresonate the barrier. And before then, in Grand Chokmah.

Hmm... maybe that's it.]

...It's also supposed to offer some sort of healing properties to the one carrying it, if it's compatible with their fonon frequency. If you feel anything when you hold it, that's probably it.