White Diamond - The leader

White Diamond, or White for short, is a Homeworld Gem and the leader of the Great Diamond Authority that formerly ruled over the Gem race. She held supreme authority over all Gems, including her fellow Diamonds, and was seen as a being beyond the rest of her kind. For eons, White Diamond remained on the Gem Homeworld, secluded inside her ship, until she finally left Homeworld to help Steven Universe heal the corrupted Gems. She made her debut in "Legs From Here to Homeworld".

 "Now the impurities you've encouraged in them are gone! Now they are brilliant; now, they are perfect; now, they are me!"

Yellow Diamond 

Yellow Diamond, or Yellow for short, is a Homeworld Gem and a member of the Great Diamond Authority that formerly ruled over the Gem race. In particular, she commanded her now-disbanded military and oversaw Gem production on her colonies. She was first shown in the extended theme song before making her official debut in "Message Received".

Blue Diamond

Blue Diamond, or Blue for short, is a Homeworld Gem and a member of the Great Diamond Authority that formerly ruled over the Gem race. She first appears in a flashback in "The Answer" and debuts in person in "Steven's Dream".

As a result of Era 3, Blue Diamond now uses her powers to bring joy to other Gems through her clouds, created from her vaporized tears, which make someone happy once coming into contact with them.

"Back before you came into my life Steven, I wanted everyone to feel the pain I felt. I realized I must make up for my awful behavior by bringing joy to others."

The Great Diamond Authority was the matriarchal body of Gem leaders. Its membership consisted of White Diamond, Yellow Diamond, Blue Diamond, and, prior to Era 2, Pink Diamond.

They were once the absolute ruling power over the Gem Homeworld and its remaining active colonies until the beginning of Era 3 and their replacement through democratic elections.

Despite this, the Diamonds still seem to retain their status as leaders on Homeworld, even if to a lesser extent. Even still retaining some influence over Homeworld, the Diamonds have begun dedicating themselves to helping Gems through their new specialties.