The Great Diamond Authority

"Each Gem is made with a purpose: To serve the order of the Diamonds. Those who cannot fit inside this order must be purged."

The Great Diamond Authority is the matriarchal body of Gem leaders who rule in absolute power over Homeworld and the existing Gem colonies. Due to their connections to various threats and tragedies, they can be considered the main antagonists of Steven Universe.

Symbol
The symbol of the Great Diamond Authority appears on many Gem structures, and has changed over time. On structures built prior to the Rebellion, the symbol consists of four diamond shapes: White (top), Yellow (left), Blue (right), and Pink (bottom). On modern structures,as well as the partially-healed Centipeetle's drawings seen in Monster Reunion, the Pink symbol has been removed, leaving only the White, Yellow, and Blue shapes (now depicted in the form of overlapping triangles).

Yellow Diamond
One of the Diamond leaders. She is the superior of Jasper (As well as the former superior to Peridot), she is also the superior to countless other Gems made for her. She commenced the Cluster project when Earth was deemed unfit for a colony, due to the Rebellion.

According to Peridot in "Hit the Diamond" Yellow Diamond is in complete control of the Gem Homeworld's military.

Subordinates

 * Peridot (formerly)
 * Jasper
 * Yellow Diamond's Pearl
 * Ruby (Doc)
 * Ruby (Eyeball)
 * Ruby (Army)
 * Ruby (Navy)
 * Ruby (Leggy)

Blue Diamond
One of the Diamond leaders. She was the superior of Ruby and Sapphire, and many other Gems "made" for her. She was once the supervisor of the colonization of Earth.

Subordinates

 * Sapphire (former court member)
 * Unknown court members
 * Ruby (formerly)
 * Other Rubies
 * Blue Diamond's Pearl

White Diamond
One of the Diamond leaders. Details about her are unknown, except for the fact that her white symbol appears alongside those representing Blue Diamond and Yellow Diamond on both ancient and modern Gem structures. Her name is never stated, but due to consistency and naming pattern, it's most likely White Diamond.

She appears to have spiky hair and her gemstone is placed on her forehead. According to her mural in the moon base she had the most colonies before the Gem War, it is unknown how many she has now.

Pink Diamond
A former Diamond leader. Details about her are unknown other than that she was in charge of the Earth colony and that her pink symbol is present on the majority of ancient Gem structures, but absent on structures dating as early as the Pyramid Temple, as she was shattered by Rose Quartz at some point during the Rebellion. She also has a mural in the moon base, along with all the other diamonds.

Pearl's space suit, as seen in "Space Race" and "Back to the Barn", has a pink symbol on its chest. This suggests that Pearl was previously involved with this Diamond prior to her destruction. Sardonyx also sports pink diamonds on her feet, this most likely comes from Pearl.Judging from the fact Jasper (who, as explained by Peridot, was created halfway or so through the war) was originally assigned to her, and that centipeetle drew only 3 diamonds when recounting the end of The Rebellion (which lasted 1000 or so years and occured roughly 5750 years ago), one can roughly estimate that she was shattered between 6250 to 5750 years previously, somewhere half way or so through The Rebellion.

According to Ruby (Eyeball), Pink Diamond was shattered by Rose Quartz. This could be supported by Jasper's final words to Steven before corruption, "Earth… is a prison. And I got out because I’m better than this place. I only came back to finish you off. You can’t manipulate me Rose Quartz!... I’ve been fighting since I broke free from the Earth’s crust! Because of what you did to my colony, because of what you did to my planet, because of what you did to my Diamond! My Diamond... Your Diamond... Pink Diamond!"

Past Subordinates

 * Rose Quartz
 * Jasper
 * Ruby (Eyeball)

Influence
The Great Diamond Authority has great influence on Gems of lower caste and members of their courts. Each member of a Diamond's court wears a diamond emblem on their outfit; the color of the diamond emblem worn by followers depend on the Diamond. Members of a Diamond's court are known to show respect, such as Sapphire calling Blue Diamond "my Diamond". Some members are passionate and enthusiastic about working for a Diamond. For example, Peridot exclaims "I would never forsake the gem I was made for!". Gems who serve under the Diamonds will do anything to avenge their Diamond, as seen in Jasper's case in "Earthlings".

Trivia

 * The current symbol might be a reference to The Triforce from The Legend of Zelda, as they both are three triangles that form a symbol.
 * The former symbol resembles the NFPA diamond, with slightly different colors (one of the diamonds is red instead of pink) and with the colors' positions switched.
 * The Great Diamond Authority was yet another theory created by Ronaldo Fryman that was proven true.
 * He claimed his theory in the first season episode "Keep Beach City Weird", and Peridot confirmed the matriarchs' collective name in the episode "It Could've Been Great".
 * It is revealed in "Monster Reunion" that the Diamonds were the ones responsible for corrupting almost all remaining Gems on Earth.
 * Centipeetle only drew three diamonds when she was asked who caused the damage, so one of the diamonds wasn't involved with this weapon, presumably Pink Diamond, who is absent in symbols after the Rebellion. This was because Pink Diamond was shattered at the beginning of the rebellion, while the damage was caused at the end of it.