It happened so quickly. Basically a tumblr user came with the (probably tongue-in-cheek) hypothesis that Onion is THE Yellow Diamond mentioned by Jasper. Because he lacks ears.

Then it evolved to something more: doesn't talk (with normal words, anyway); general odd behavior in "Onion Trade"; when Steven mentions some humans living in the planet ("Marble Madness") mentions Onion and right then says "I think"; real life yellow diamonds are a synthetic mineral, made with something called carbon onions.

Counterarguments: Jasper says about bringing Yellow Diamond, implying she's in a place (Gem Homeworld, presumably) that isn't Earth; Onion has a human father and brother (Sour Cream); IF Onion is Yellow Diamond, she would already killed the Gems living in Beach City; in "Beach party" Amethyst can be seen with a ear.

Countercounterargument: It was stated repeatedly that Gems can shapeshift.