Okay I think I've worked out most of the details for the gem au. The main changes from SU canon are that gems have names (based on their code number, so like Garou's code would be like 844-GRO, something like that), and can have any gender identity. There are no crystal gems either, or at least none that Homeworld knows about so it doesn't effect the story. And if the diamonds ever come into play they'd probably be different characters.
So!
King is a jasper, utterly flawless, he popped out perfect and everyone expected great things of him. So did King, until his first battle when he was nearly shattered and hurt so badly the trauma left a permanent mark on his physical form. The battle was still won, to his surprise, and as part of the spoils of war King is given a pearl that had belonged to the leader of the opposing side.
Pretty quickly King realizes that the pearl is the one who actually defeated his opponent, which shouldn't be possible. Pearls can't turn against their owners, can they? But Saitama is doing his best to keep his head down for now, and King likes having someone he doesn't have to hide from, so he eventually admits what he's guessed and Saitama confirms it. That guy was a bad owner, Saitama says, and won't say any more.
There's a new planet and a new fight, this time merely against the local non-sentient wildlife. King tries, fails, and Saitama has to rescue him. They come to an agreement that Saitama will do the fighting and King will be the face of the operation. For a while things are as happy as they can be on a planet full of stuff that wants to kill you.
It's harder back on Homeworld. There are more expectations there. Nobody can figure out why Saitama looks so plain, and doesn't open doors for them or entertain on command. And there's a growing rebel movement that King keeps being told he'll probably have to help put down soon, which is worrying for several reasons.
Saitama learns about the rebels by accident. A blue chalcedony (rare and highly prized) was mistaken for an off-color when he stepped in to defend a smaller gem. Saitama saves Genos just before he's shattered, but Genos poofs from his injuries and Saitama simply puts his gem somewhere safe and leaves him. Genos spends weeks looking for that strange plain pearl.
And then once again, King is shipped out to fight. He's given his own troop to command this time... including a rare blue chalcedony.
