Notes:

My personal theory: as the confidant of a Diamond Spinel could have overheard where the Authority kept its superweapons. Last-resort stuff like the Rejuvenator and the Injector would have probably been kept in the same place, so it would have been one stop on the way to Earth. And with the Authority disestablished the Rubys and Quartzes guarding that stuff are probably long gone – like Babylon 5 after the Shadows left for the Galactic Rim, there's lots of Doomsday stuff lying around ripe for the taking…