The Cluster

"But now it is going to emerge and nothing can stop it, and we'll all be shattered!"

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"The Cluster" is a massive Cluster Gem and a currently unknown antagonist which, while little is known about it, is foreshadowed to play a big part in the overarching plot of the series.

History
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.

The Cluster 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 "emergence" of The Cluster would be a catastrophic event that would "shatter" (kill) everyone. In "When it Rains" Peridot confirmed The Cluster to be a giant forced gem fusion at the core of Earth comprised of millions of gemshards. She mentioned that it will be much bigger than Earth and will destroy the planet while emerging, though it is possible to stop it from hatching.

Trivia

 * Peridot is utterly terrified of "The Cluster" as she knows what it is and what it is capable of.
 * She often uses her knowledge of "The Cluster" as leverage to prevent the Crystal Gems from harming her, but to no avail.
 * Before "Catch and Release", the Cluster was assumed by fans to be the former Crystal Gem fusion experiment seen in "Keeping it Together".