The Great Diamond Authority

"Are you joking me?! The Diamonds are the Gem matriarchs! Together they make up the Great Diamond Authority that governs Homeworld and all the outlying colonies! We live to serve them."

"The Great Diamond Authority" is the matriarchal body of Gem leaders who rule in absolute power over Homeworld and the existing Gem colonies.

Sigil
The symbol of the Great Diamond Authority appears on many Gem structures, but has changed over time. On structures built prior to the Rebellion, the symbol consists of four diamond shapes: white (top), yellow (right), blue (left), and a pink one (bottom). On modern structures, the pink symbol has been removed, leaving only the yellow, blue and white shapes (now in the form of overlapping triangles), suggesting that the Authority has been reduced in membership from four to three during that time.

Yellow Diamond
One of the Diamond leaders. She is the superior of Jasper, and many other Gems "made" for her. She is the former superior of Peridot, now a Crystal Gem. She commenced the Cluster project when Earth was deemed unfit for a colony, due to the Rebellion.

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.

Unknown Diamond (White)
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. Due to a naming pattern (color of the Diamond) there is a possibility that this Diamond is called White Diamond.

She appears to have spiky hair and her gemstone is placed on her forehead.

Unknown Diamond (Pink)
A former Diamond leader. Details about her are unknown other than the fact that her pink symbol is present on ancient Gem structures.

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 "removal" from the Authority. Sardonyx also sports pink symbols on her feet, this most likely comes from Pearl.

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 was made for Yellow Diamond!".

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".


 * So far, the reason why the Diamond Authority was reduced to three members remains unknown.