"Cluster Gems", also known as Gem Mutants and Artificial Fusions, are forced fusions of former Crystal Gems. They were supervised by Peridot and other Homeworld Gems created to serve as prototypes for the real Cluster. As of "Nightmare Hospital", new Cluster Gems have started emerging and become increasingly mobile and violent. These experiments were created in the Kindergarten.
In general, Clusters take the form of limbs joined together to other appendages. The limbs are of mismatched colors, such as dark and light blue, rust, violet, olive green, mint gray, pale green and dull yellow, indicating that they originally belonged to different Gems.
Referred to as "Mini Cluster Gem" in the model sheets, this cluster consists of a purple and blue arm fused together.
A cluster that consists of a blue hand and red foot fused together.
Referred to as "Hand Hand" in the model sheets, this cluster consists of a purple and green hand fused together.
Referred to as "Hand Cluster" and "Hand Cluster Gem" in the model sheets, this giant variant of a cluster appears to be comprised entirely of limbs conjoined together in a single hand-and-forearm-shaped body, wrapped in what looks like a dark grey cloth. At its core, the fusion is comprised of four different shattered Gem Shards that are physically fused together. It possesses four large eyes. Its irises and visible Gem Shards are green, yellow, blue and purple.
A cluster that consists of a green and red foot fused together.
Referred to as "Arm Leg" in the model sheets, this cluster consists of a yellow leg and a green arm fused together.
A cluster that consists of a black arm with a red and blue hand fused together.
Referred to as "Patient 1" in the Nightmare Hospital episode credits, this cluster has six arms and two legs. It takes a more human form, however two arms take the place of where a head would be and has two arms sticking out of its right side. Its right leg also ends with a mismatched lower leg that is much smaller than the part above. The limbs are in various colors such as orange, teal, gray, blue, and a dull purple. The location of its gem is unknown, assuming it is visible.
Referred to as "Patient 2" in the "Nightmare Hospital" episode credits, this cluster has three arms and two legs. It has two large lumps where its face would be. Its left arm acts as a leg and is very long, huge and muscular. Attached to it is two parallel hands. It has two right arms. One of its legs is very small, not allowing it to act as a real leg. It comes in many mismatched colors, including yellow, spring green and dull purple.
A cluster that has 6 feet but uses 4 of them as arms. Two of the feet are dark purple and the rest of its body is dull red. On the bottom it has a vertical mouth that opens to reveal a single eye. It also has a dull yellow arm as a tail.
Steven and Garnet encounter Cluster Gems in the Kindergarten, the Hand Cluster and six smaller Cluster Gems. The appearance of the Clusters is so horrifying to Garnet that she becomes unstable to the point where she almost un-fuses. Ruby, through Garnet, states she believes the Cluster Gems were created as punishment by Homeworld for The Rebellion. The Cluster Gems are eventually "poofed" by Garnet and currently reside within one of her bubbles in the Burning Room.
Two Clusters are sighted across roads and taken to Dr. Maheswaran's work place, mistaken as victims of car accidents and treated by doctors at the Hospital. Later, they break into the halls and try to attack Steven, Connie and Dr. Maheswaran, only to be poofed by Connie with Rose's Sword and then bubbled by Steven.
Peridot and Steven are assaulted by several of them when attempting to escape the Kindergarten before Steven forms a bubble and they are rescued by the Crystal Gems. This is the second time where the Cluster Gems are shown to have facial features (i.e., mouth, eyes); the first time being "Keeping it Together".
Only twenty out of the 180 cluster prototypes have been seen.
Clusters, along with Rainbow Quartz, are the only Gems that have never spoken in the series (omitting the screams emitted by them).
Coincidentally or not, if one excludes Hand Cluster's thumb (which is covered by a boot), it has a glove covering every finger but the middle, while Garnet has a glove covering her middle finger.
The blue "leg" off to the left of the Hand Cluster appears to have the same boots as Amethyst and Ruby.
There appears to be no red Gem Shard in the Hand Cluster, despite the red leg and arm present in it.
The silhouettes of the Hand Cluster members have traits similar to other Gems on the show. For example, their hairstyles are similar in appearance to that of Pearl's haircut from the Pilot.
It should be noted that when the Gems' silhouettes appear, each "body" seems to have a similar body type and hair style.
This was later confirmed to be an animation error by Matt Burnett via Twitter.
Each of the Hand Cluster's eyes has a defined plica semilunaris on either side of it, not seen elsewhere on eyes in the show.
The Hand Cluster and Garnet's gemstones share the same placement, both having their gems in the center of their palms.
The rock cylinders used to hold the immature Cluster Gems resemble rock core samples.
In "Nightmare Hospital", it is mentioned that they have no vitals or heartbeat, a fact that may also pertain to regular Gems as well, given their fluid physiology.
In the episode "When it Rains" two Cluster Gems with actual heads, mouth and eyes are among the group attacking Steven and Peridot.
A total of 180 prototypes are seen on the screens in "When it Rains".
Clusters share many similarities to Frankenstein's monster from the famous film adaptation of Mary Shelley's novel of the same name.
Both are made out of body parts of different deceased individuals, acquired via grave robbing.
They are unnatural creations which were created through scientific means.
Both are represented as tragic, yet horrifying figures.
The malformed appearances and multi-limbed forms of the Cluster Gems give them a striking resemblance to the Necromorphs from the Dead Space series.
Their grotesque amalgamations of body parts also bear resemblance to John Carpenter's The Thing, as well as its 2011 prequel, which does show arm-based creatures.
The two-headed Cluster Gem resembles the Charger in the Left for Dead video game series in terms of anatomical design.
The Hand Cluster's cloth makes it resemble a mummy.
A cluster of gemstones is a result of gems that fuse together due to extensive pressure from being underground. In most cases clusters are usually fusions of the same gemstone; however, in very rare cases, it is possible for clusters to form from different gemstones.
The Hand Cluster's gemstone is a mix of four different gems joined into a single gemstone. It's located on the center of its "palm".
A smaller Cluster taking the form of two forearms has two Gem Shards fused together into a V-shape, with smaller pieces stuck around the joint. One is blue, the other is purple. The shards are internal.
The gemstone of the six-armed Cluster Gem - a large red gemstone with squarish facets with shards of smaller purple gemstones stuck to the side.
The gemstone of the two-headed Cluster Gem. Two large gemstones with squarish facets, one yellow and one blue, with smaller shards attached to them. The gemstone is located between the two faceless "heads", with the yellow gemstone sticking out and the blue gemstone mostly hidden.
The gemstone of the Cluster with a Light Green head that has one eye and mouth on it's face, a light blue body and left hand, a big green foot as it's right hand, and purple lower body.
The gemstone of the Big White Hand Cluster.
The gemstone of the Blue Hand with seven fingers and a foot Cluster.
The gemstone of the cluster with a round violet body, three blue limbs, and an eye.
The gemstone of the small cluster with a blue hand and red leg.
The gemstone of the cluster with a big blue hand and pink foot.