The "Cluster" is a massive Cluster Gem that was first mentioned in "Jail Break" and first seen in "Gem Drill". It is composed of millions of gem shards and, in its emerged form, would be larger than the Earth. It was bubbled by Steven in "Gem Drill" and remains in the Earth's lower mantle.
The Cluster was first mentioned by Peridot in "Jail Break". Arriving on Earth and checking on it was the primary objective of her mission to Earth, with Jasper as her escort and Lapis Lazuli as her informant.
It was later mentioned in more detail in "Catch and Release", when Peridot was cornered and finally captured by the Crystal Gems. When she was released by Steven, she reveals that the "hatching" of The Cluster would be a catastrophic event that would destroy everything on Earth.
Peridot explains that after Homeworld no longer saw Earth as a viable colony to mass-incubate Gem warriors, they started doing experiments with the Gem shards, fusing them together to create an artificial fusion as a type of "Gem geo-weapon". She reveals in the Prime Kindergarten Control Room that the forced Gem fusions they were fighting were prototypes for the Cluster, which was revealed to be a giant forced Gem fusion at the core of Earth composed of millions of gem shards. She mentioned that it will be much bigger than Earth if it emerges and takes its physical form, and will destroy the planet once it does so.
Peridot mentions the Cluster to the Crystal Gems, revealing the details of what the Cluster actually is. The Crystal Gems then decided to build a drill to try and reach the Cluster before it finishes its incubation period.
The Cluster was revealed to have been inserted in the Beta Kindergarten into the mantle approximately 2,500 units down relative to Greg's Storage Shed.
After taking the Diamond Communicator, Peridot argues in favor of the plan to terminate it to Yellow Diamond. Yellow Diamond replies she couldn't care less that it will destroy the Earth, and orders for Peridot to let the Cluster continue to grow.
Earthquakes begin rocking Beach City indicating that the Cluster is only hours away from taking form. Steven and Peridot drill to the center of the Earth where the Cluster is located, but are unable to pierce the surface of it. As Peridot tries to destroy it, Steven sees faces and hears the voices of the gem shards comprising the Cluster, and starts to talk to it as he falls unconscious. He convinces it not to form and, instead, to bond with itself. The Cluster begins to uncontrollably take form, however, and Steven attempts to bubble all the gem shards. He eventually wakes up and notices the gem shards inside the Cluster are bubbling each other. Steven finishes it off and merges all the small bubbles into one immense bubble, completely encasing the Cluster and preventing it from destroying the Earth.
- Peridot is utterly terrified of the Cluster, as she knows what it is and what it is capable of. Before her truce with the Crystal Gems, she often used her knowledge of the Cluster as leverage to prevent them from harming her, but it functioned as a stalemate at best.
- The plan to hollow out the Earth was foreshadowed by Ronaldo's conspiracy theories in "Keep Beach City Weird".
- Peridot refers to the Cluster as a "Gem Geo-weapon". This is a play on the term "Bioweapon", Bio- referring to organic life while Geo- referring to rocks and minerals. This is appropriate, seeing as how Gems are a race of sentient gemstones and are thus inorganic.
- In "It Could've Been Great", a map on the moon base reveals that the Cluster is currently located somewhere around Death Valley in southern California/southern Nevada.
- The Cluster bears a great resemblance to Lavos, the main antagonist of the Chrono Trigger video game; as both of them are extraterrestrial creatures that will initiate an apocalyptic event if they are released/awakened unless a group of heroes stop them.