Surprise
The first mission Steven goes on with the Diamond Defectors is to meet Jadeite, the gem in charge of Earth.
Jadeite, Steven learns, visits Earth every few years to make sure the Diamond Defectors aren't killing humans and that they haven't attempted to build any injectors. She also beats up Jasper a lot, which Jasper seems to respect judging by how she talks about her. If Steven didn't know better he'd say she had a crush. There's even a photo album in the temple full of snapshots of Jadeite pinning Jasper, all taken by Peridot.
Fittingly, Peridot makes sure to have her camera on hand when they get on the warp pad.
The thing is, Jadeite hasn't visited Earth in nearly twenty years, which means that she has no clue Diamond has given her gem to Steven, or that Steven exists at all.
"The element of surprise is very important." Jasper says, right before the warp pad activates.