Homeworld Gems

The Homeworld Gems, also known as The Homeworld, are the original group of gems who came to and exploited Earth, causing the Crystal Gems to splinter and oppose them. They were first alluded in the episode, "Serious Steven" and mentioned in the episode, "Mirror Gem". They serve as the potential true antagonists of the Cartoon Network TV series, Steven Universe.

Before Steven Universe
6,000 years before the events of the series, gems came to Earth and spread their influence to it, creating gem landmarks and monuments. One of these landmarks was the Kindergarten, a "breeding ground" for creating gems. In an unknown amount of time later, it was shut down by Rose Quartz and her followers. 1,000 years later, a war had started between Homeworld and Crystal Gems, resulting in a victory for the latter. The Homeworld Gems then returned to their world and, over the course of 5,000 years, developed advanced technology like gem destabilizers and improved gem warships for their second attempt to take control of Earth.

Season 1
After thousands of years, the Homeworld Gems send a scout drone called a "Red Eye" to Earth to detect gems but was promptly destroyed by the Crystal Gems, without reporting any activity. After that, they send Peridot and her seventy-nine Flask Robonoids to repair Earth's Homeworld Warp, only to have her leave, resulting in the Homeworld Warp being destroyed by Garnet. They again send Peridot and her Plug Robonoids to the Kindergarten to reactivate it, only to be thwarted once again by the Crystal Gems. However, Peridot did return with knowledge of Earth-resident gems. In response to the Crystal Gems, they send Peridot, Lapis Lazuli, and Jasper on a mission to capture the Crystal Gems. In the process, they discover that Steven has Rose Quartz' power, and decide to bring him to their supposed superior, Yellow Diamond. Later on the Gem Warship, Peridot mentions that they needed to find an entity called, "The Cluster". The Crystal Gems manage to escape, destroying the warship and sending it and everyone aboard crashing beside the Crystal Temple, with the exception of Peridot who was ejected somewhere on Earth's surface prior to the warship's entry into Earth's atmosphere. Back at the crash site, the Crystal Gems manage to survive the crash, but suddenly, Jasper emerges from the rubble, weak and exhausted until she sees Lapis Lazuli and convinces her to fuse with Japser, forming Malachite, only to have Lapis take control of the whole fusion dragging herself and Jasper to the bottom of the ocean. In the aftermath, the Crystal Gems celebrate, but Greg brings up the possibility of more Homeworld Gems retaliating, meaning their fued with the Homeworld isn't over yet.

Season 2
Steven and the Cool Kids accidentally discover Peridot's damaged escape pod in a crop field. When they investigate it, Peridot had already left with her whereabouts left unknown. In an attempt to find her, the Crystal Gems travel to the Kindergarten. Steven discovers Peridot leaving Facet Five of the Kindergarten, causing the other gems to chase her. Peridot uses her fingers like helicopter blades to fly away with Pearl and Amethyst in pursuit. Meanwhile, Steven and Garnet enter Facet Five and find fused gem shard limbs, and the Cluster, a forced fusion of gem shards combined into a gem monster, as a form of punishment for the fallen soldiers of Rose Quartz's rebellion. Garnet defeats the monster, and Peridot escapes.

Trivia

 * It's stated by Pearl that the Homeworld controls multiple planets.
 * They are implied to be ruled by a group of three diamond gems, indirectly nicknamed the "Great Diamond Authority" by Ronaldo Fryman and later by fans.
 * The flare launched by Peridot's escape pod in the episode, "Joy Ride" may have alerted the Homeworld.
 * Their reason for creating the Kindergarten remains unknown.
 * In the episode "The Message" Lapis Lazuli remarks that Homeworld is vastly different and more technologically advanced than she last remembered it.
 * It's unknown if they came to Earth with the intention of exploiting it, or if they originally came there to influence it and decided much later to exploit it.
 * It's implied that they may have had some involvement with the Geode and the Gem Monsters.
 * The Homeworld shares some similarities with The Irken Empire from The Invader Zim series.