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Gem Shards are the remnants of a shattered Gem's gemstone that first appeared in "Frybo". They also appeared in "Secret Team", "Keeping it Together", "Nightmare Hospital", "When It Rains" and "Gem Drill". They are the shattered remains of broken Gems that still contain remnants of the original Gem's form. The shards have different abilities, such as possessing material objects. The Gem shards can be forcibly fused together to create a mutant Cluster Gem.
Pearl tried to explain to Steven that "These shards have a powerful partial consciousness that has been harnessed by Gems throughout history to create semi-sentient drone soldiers with the capacity to follow basic orders. Gems once created an army of these drones but found their obedience waned as the shards overdeveloped inside their uniforms and turned on their commanders. You see, any shard inside any sort of container could become a monster!" An example of this is when Frybo turned on Steven and Peedee Fryman, which is why the Gem Shards must be kept away from all clothing and other items to avoid other dangerous situations.
Pearl, Amethyst, and Steven accidentally popped a Rose Quartz bubble containing a mass of mixed Gem shards, which regenerated into non-sentient disembodied arms and legs. After defeating and collecting the scattered Gem Shards, Steven bubbled them and they attempted to keep it a secret from Garnet, but failed.
It is implied by Pearl that these shards have unique properties (such as the ability to regenerate in a partial state, something only Cluster Gems have been observed to do) due to being bubbled by Rose.
While in the Kindergarten, it is revealed that Gem Homeworld is forcing the gem shards of fallen rebel gems from the war to fuse. The shards can take on a form, but the generations only result in limbs spliced together in unnatural ways. The former Crystal Gems that comprise the Cluster Gem mutants can regenerate, but the process causes them obvious pain. Whether or not the forced fusions regenerate by will is unknown; in their debut moments, their constituents are seen trying to escape the fusion, but that may just be spontaneous regeneration like that of the Gem Shards in "Secret Team".
Two more Gem mutants appeared, having been mistaken for car accident victims and admitted to the Hospital as patients. They then attack Steven, Connie, and Dr. Maheswaran before being poofed by Connie using Rose's Sword. These Gem mutants are much more mobile and violent than those seen in "Keeping it Together".