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There are three, Count them! Three different gems (excluding Rose) on the mural. Any of which could be Yellow Diamond. (personally I favor the one on the left.) Each on of them Could be a "Diamond" just not "Yellow Diamond" (White Diamond, Blue Diamond ect.) Also look here.

And Here

Notice how there are three Colors (Yellow, White and Blue) in the bottom one. 3 Gems, 3 colors.
That symbol represents the "Bad" gems or the Home-world Gems, but more importantly their leaders.

My theory is the gem Rose is Fighting Is White Diamond. The "Supreme" leader of the Home-world. (why White? well most people Think that White diamonds are the most precious Gems so that could easily the reasoning behind that.) We are never the Specifics on how the Crystal gems won earth. My guess? Rose beat their leader and the rest all ran scared.

Will she be Buu Saga Vegeta? The best Vegeta?