If you looked closely on the episode "Prison Break", you'll notice on the floor of the spaceship where Garnet and Jasper are fighting, there are three diamonds in a triangle.
So what?
In one of the newer episodes, when they go to the moon base, there is a pattern on the ground with four diamonds on it.
I think they banned fusion because somehow, two gems fused and overthrew one of the diamonds.
That's why Blue Diamond was so afraid, even angry at Sapphire and Ruby fusing.
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Steven Universe Oneshots
FanfictionHello and welcome to a book consisting of Steven Universe! The chapters range from headcanons, to short stories, and mostly angst. And ships. Have fun reading! (half of this is angst i lied, lapis is my son, garnet should be ruby x sapphire's ship...