Ok, so I came up with a theory.

So, White Diamond can't tell the difference between Steven and Pink Diamond and treats Steven, who has Pink Diamond's gem but doesn't look like her, exactly like Pink Diamond. What if White Diamond is blind, and can just feel gemstones somehow (sorta like the shattering robonoids)? After all, her eyes have white pupils and gray irises, which gives the impression of her possibly being blind.

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