Cluster Gems

"Cluster Gems" are forced fusions of former Crystal Gems. They were supervised by Peridot, and created through unknown means. As of "Nightmare Hospital", new Clusters have become increasingly mobile and violent.

Appearance
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.

"Marble Madness"
The Clusters make a semi-hidden cameo when Peridot is accessing the Facet Five control room.

"Jail Break"
When Peridot and Jasper are at Sapphire's prison cell, Peridot exclaims, "We can't leave yet! The whole point of coming here was to check on the Cluster!".

"Keeping it Together"
The appearance of a Cluster was so horrifying to Garnet that she became unstable to the point where she almost un-fused. Ruby believes the Clusters were created as punishment by Homeworld for The Rebellion. It was eventually "poofed" by Garnet and currently resides within one of her bubbles in the Burning Room.

"Nightmare Hospital"
Two Clusters are sighted across roads and taken to Dr. Maheswaran's work place, mistaken as victims of car accidents and were treated by selected doctors at the Hospital. However, they broke through the halls, tried to attack Steven, Connie and her mom, only to be poofed by Connie via use of Rose's Sword and then bubbled by Steven.

Trivia

 * Clusters, along with Rainbow Quartz, have never spoken in the series (omitting the screams emitted by them).
 * Coincidentally or not, if one excludes the 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 appears to have the same boots as Amethyst and Ruby.
 * There appears to be no red Gem Shard in the main Cluster, despite the red leg and arm present in it.
 * The silhouettes of the 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, implying that the attempted forced fusion may have been somewhat successful, before they reformed into a limb-comprised hand.
 * When the Cluster's gem was sent to the Burning Room, its gem reshaped itself to appear like the Centipeetle Mother's gem.
 * This was later confirmed to be an animation error by Matt Burnett via Twitter.
 * Each of the Cluster's eyes has one plica semilunaris on either side of itself.
 * The 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.
 * The six-armed Cluster has a feminine voice, evidenced by its moans and breathing noises it makes in "Nightmare Hospital".
 * Its voice becomes more deep, garbled, and demonic-sounding throught the episode.

Cultural References

 * Clusters share many similarities to Frankenstein's monster from the Mary Shelley novel of the same name.
 * Both are made out of body parts of different deceased individuals, acquired via grave robbing.
 * This is not true in the book, where the means of making the monster were never specified, but is true of most adaptations.
 * They are unnatural creations which were created through scientific means.
 * Both are represented as tragic, yet horrifying figures.
 * The cylindrical containers may be a reference to the 2012 sci-fi horror movie Prometheus.
 * Their malformed appearances and multi-limbed forms of the Clusters give them a striking resemblance to the Necromorphs from the "Dead Space" series.
 * The two-headed cluster gem resembles the Charger in the Left for Dead video game series in terms of anatomical design.

Gemology

 * 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.