Black diamond

This is Era 4 and While Steven and the crystal gems were exploring a abandoned Colony they discover Deep within the sub-layers of the Reef, Steven and Pearl find a door sealed with a dark, cold energy. As the door slides open, Steven’s flashlight hits the wall, revealing the fractured purple and black stone.
Steven: "I thought I knew all the murals... who is that?"

Pearl: (Her hands fly to her mouth, her form flickering with a static-like glitch) "She... she shouldn't be here. She was the one who came before White. The one who taught the Diamonds how to 'hollow' a planet. We were told she was a myth—the 'Shadow of the Authority' that took the Gems who wouldn't fit."
Steven: "But she’s holding planets. She had a court. What happened to them?"
Pearl: "When the light of a Diamond fails, Steven... she is the darkness that remains."
and From space, the planet looks like a solid sphere of obsidian, but it is actually a cage of interlocking tectonic plates that block out all external starlight.
The Interior: The civilization lives on the inside of the crust, looking down into a core that isn't a sun, but a singular, massive black diamond that pulses with a cold, violet rhythm.
Atmosphere: The "air" is thick with the same dark, ink-like smoke seen in her power demonstration, acting as a sensory network for the Diamond herself.

and her court is

black pearl A silent, silhouette-like gem with no face, only a single white eye on her chest. She moves through shadows like a 2D drawing. Obsidians:Heavy-set sentinels wearing visors similar to Black Diamond's. They specialize in "erasing" traces of rebellion. Onyxes: High-ranking aristocrats with the power to manipulate gravity or light, creating "void zones" where no one can see or hear.Shadow Agates:Terrifying quartz commanders who use whips made of solidified darkness to maintain order in the Hollow World.