Fake news, the Steven Universe AU.
I thought about doing a bunch of pretty-minerals-and-jewels research to decide what gems they would be…then remembered that the Gems’ names aren’t (all?) individual personal names, they’re types/castes. (“I’m just a Pearl,” etc.)
So: Jon is a Sapphire, “Stephen” is a Ruby!
Now, the next question is, are the fusions standardized? That is, if any Ruby and any Sapphire fuse, will they always form a Garnet? If yes, then ignore the labeling on Colbewart here. But I want their fusion to have more of a purple tint, to emphasize Jon’s strong influence, and that’s easier to get with a Spinel.