Inspired by BabyBlue2945

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.

We know from Peridot-

That Yellow Diamond's court like to use electricity to battle.

The Ice was Blue Diamond's.

We know from Sapphire-

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.

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.