The Crystal Gems' Entire Backstory Explained

Blue Diamond invades Earth to take it back from the upstart rebel leader Rose Quartz, fully unaware that Rose is Pink Diamond. In one of the ensuing battles between Rose's band of Crystal Gems and Blue Diamond's shock troops, a fateful accident earns the Gems a new and valuable general.

Two of Blue Diamond's troops have been developing feelings for each other. However, they are forced to keep their attraction a secret, as fraternizing between different classes of Gems is considered vile by Gem society. Gems are able to merge with other members of their class to create more powerful "fusions." However, two different Gems merging is met with scorn and disgust.

While attempting to save her secret paramour from an attack by Pearl, a Ruby accidentally merges with a Sapphire creating the fusion known as Garnet. Combined, she has Sapphire's ability to see the future and Ruby's incredible fighting prowess. Still, rather than seeing this as a useful asset, Blue Diamond reacts with revulsion to their fusion. She orders that Ruby be "shattered" for fusing with a gem of higher status, in effect laying out a death sentence. Rather than submit to the sentence, Sapphire and Ruby join the rebellion on Earth, living in fusion as Garnet, and quickly become one of the Crystal Gems' most valuable fighters.