@Crimson Locks
To fuel your hypothesis of Rose's motives, see the season 1 finale, where Steven is harmlessy attacked by the gem destabilizer and he can nullify the cells tailor-made for gems, he being half-human, or more exactly, not being a full-on gem, must be pretty much the cause.
Depends on Rose's foresight, of the Homeworld creating tech with the specific purpose or neutralizing gems.
@Onion Syrup
Nope, there are plenty of others. One of the Ben 10 sequels retconned Gwen's magic into a skill inherited from having an alien grandmother. The fandom didn't like it, it seems.
Other examples include Madoka Magica (Kyubey's origin), Jojo (part 5 tells where the Stand arrows come from and what they do, but remember that we are talking about Araki's storytelling), Evangelion and the Angels and Eva units, Lovecraft's works often includes creatures that are explicitly extraterrestrial beings.
(note that out of these example, i just watched/read MadoMagi, Jojo, and Eva)
There are even more example, tvtropes has a page called Doing in the Wizard.