So in "Lion 2: The Movie" Lion takes Steven and Connie to Rose's private arsenal. There they battle a Robot Shooty thing that Lion doesn't seem to afraid of. I believe Rose used to battle the tri-head and that the three heads were the diamonds.
The Electricity was Yellow Diamonds.
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We know from Peridot-
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That Yellow Diamond's court like to use electricity to battle.
The Ice was Blue Diamond's.
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We know from Sapphire-
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That blue Diamond's court can use ice to their advantage.
The fire was White Diamond's. Pearl can use a move called "Fireball" and this helps prove the Pearl was White Diamond's theory.
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We know there were four Diamond's yet Rose was only prepared to fight four. This means that Rose had to have been the fourth diamond.