Great Diamond Authority

"Every Gem is made for a purpose: To serve the order of the Diamonds. Those who cannot fit inside this order must be purged! To come out misshapen, to reshape yourself outside your purpose, and to defend this ruined, worthless planet is a disgrace!"

- Jasper "The Diamonds, unique in their flawlessness, sought to expand their perfection across the galaxy."

- Garnet, describing The Great Diamond Authority.

The Great Diamond Authority is the matriarchal body of Gem leaders led by White Diamond who rule in absolute power over the Gem Homeworld and its remaining colonies. They are the main antagonists of Cartoon Network series Steven Universe.

Backstory
Prior to the events of Steven Universe, there were four diamonds on the group (Yellow, Pink, Blue, and White). Later, the Diamonds began the colonization of several planets they encountered, with Earth being part of the colonization, in which Pink Diamond was in charge of completing (successfully) the colonization on the planet. However, one of her underlings, Rose Quartz, shattered her and began a revolution against the Diamonds, thus founding the Crystal Gems. Time passed and a war on earth started in which the Diamonds left Earth to its fate by the Cluster and, to make sure Rose and the Crystal Gems pay, they unleashed a corruption light across the planet which Rose and the remaining Crystal Gems (Garnet, Pearl and Amethyst) survived thanks to her shield.

Unfortunately for them, the remaining of their troops didn't get to escape and were corrupted along with the remaining Crystal Gems. Despite this, the Diamonds were aware the Cluster would eventually destroy Earth, getting rid of the Crystal Gems' base forever.

Season 1
The Diamonds' presence was completely unknown during the events of Season 1, but they are mentioned by Jasper in "Jailbreak", when she says "Yellow Diamond needs to see this...thing". Their emblem is also shown in the Gem Warship without Pink Diamond on it.

Season 2
In Season 2, the Diamonds' presence is limited with Jasper being stuck underwater with Lapis as the unstable fusion known as Malachite. Peridot is a recurring antagonist of the season and she is seen monitoring abandoned gem sites and attempting to contact Yellow Diamond and try to get off of Earth. Peridot is eventually captured by the Crystal Gems, and ends up teaming up with them to stop a gigantic, monstrous Gem Monster called "The Cluster", which the Diamonds had sent her to check on. Peridot eventually makes contact with Yellow Diamond, however, she ended up defecting from her and became a member of the Crystal Gems.

Season 3
In Season 3, more information is found out about the Diamonds. In "Hit the Diamond", a group of five ruby soldiers from Homeworld are challenged to a game of baseball and they end up revealing that their real mission is to locate Jasper and go off to Neptune after Steven lied to them about her whereabouts. After being unfused from Lapis, Jasper begins assembling an army of corrupted gems to fight back against the Crystal Gems and has them all caged in the Beta Kindergarten. After Steven, Peridot, and Amethyst find her, Jasper reveals that all gems are made to serve the the Order of the Diamonds (Great Diamond Authority) and that all gems who don't must be purged. She eventually fuses with a corrupted gem but it only lasts for a short period of time and she ends up being corrupted afterwards but not before she states that Rose had done something to Earth and Pink Diamond, her former superior and is then turned into a corrupted gem. Eventually, the five rubies return, Amethyst shape-shifts into Jasper, and they go to the moon base where it's revealed that Rose Quartz had actually shattered Pink Diamond. In "Bubbled", Steven and Eyeball are stuck in space and Eyeball reveals her intentions to kill Steven, who she thinks is Rose, to avenge Pink Diamond and return back to Homeworld as a hero but it fails and she's thrown back into space.

Season 4
In Season 4, Blue Diamond came to Earth in Korea, which is where Pink Diamond' was shattered. There Blue was mourning over how she wished she could have done something to help out Pink. She then realizes that she wasn't alone. Her Pearl brought before her, Greg Universe, who asks Blue Diamond if she was close with Pink Diamond, in which Blue Diamond replies, "Very". Greg also mentions that there is also someone he was very close with (Rose Quartz) and he misses her very much. Blue Diamond is surprised that humans could even be able to miss people they care about. She decides to take Greg with her, for she believes that The Cluster is still going to emerge, and so she intends to "save" Greg from The Cluster, as well as to save a piece of Pink Diamond's legacy. So she grabs Greg and enters her Ship and leaves to Pink Diamond's human zoo.

By the time the Crystal gems arrived to the zoo, Blue Diamond had already left the zoo.

Season 5
In Season 5, Yellow and Blue came to the trial for Rose Quartz's case from the demise of Pink Diamond.

Trivia

 * They have diamond-shaped pupils in their eyes.
 * They are possibly siblings.
 * They are all huge except for Pink Diamond.
 * Their current Symbol resembles the Triforce from The Legend of Zelda.