Gems appear to actually be a conquering alien race that sap a planet for its resources to create new gems. Steven's mom, Rose Quartz, led three other gems against the invaders before giving up her form to create Steven. Well, skip forward to whenever we are in the timeline, and the Gems are trying to invade again as a technical based Gem, Peridot, begins work to try to reconnect the Gem homeworld to Earth. After thwarting her teleportation pad efforts, she eventually just comes flying in on a giant robot hand, and with the help of a burly gem called Jasper, captures our protagonists and takes off for homeworld. Skip forward a bit.
Garnet is actually a fusion as fans have been speculating, of two characters called Ruby and Sapphire. They appear to be in genuine love, and that may have a role into why they can stay a fusion. They don't even have to do a dance to come together. Garnet fights what is effectively a Star Platinum who punches with her head, while singing a really good song. Video here:
https://www.youtube.com/watch?v=48jt_lWA7W0&list=WL&index=20