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.