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Even though we've already won the fight for Earth, we're still trying to keep it safe! There are battle-damaged Gems that are still around, and they're not like us. Something's really wrong with them. They're like horrible monsters. We have to poof their bodies away, and then keep their Gems inside of bubbles, so they can't form again and hurt anybody. Maybe someday I'll be able to heal what's wrong with them, but I'm not sure how. Mom couldn't fix them either. For now, we're keeping them safe inside the Temple.
|—Steven, "Guide to the Crystal Gems"|
Corrupted Gems, also known as Gem Monsters, are Gems who have become corrupted, assuming a monstrous form. One of the main objectives of the Crystal Gems (other than their protection of humanity) is to defeat and contain corrupted Gem monsters, and hopefully cure them.
All those monsters we fight used to be just like us! Right, Pearl?
|—Amethyst and Pearl, "Ocean Gem"|
Corrupted Gems used to be normal Gems before the events that unfolded at the end of the Rebellion. As a last ditch effort in the war for Earth, the Great Diamond Authority sent out a Corrupting Light which corrupted Gems from both Homeworld and the Rebellion that had been left on Earth. In "Monster Reunion", Nephrite described it as a "song" and remembering it seemed to be what triggered her to revert into her corrupted form.
As revealed in "Earthlings", second-hand corruption can occur to a non-corrupted Gem through fusion with a corrupted one, as Jasper started corrupting after fusing with the Snow Monster and forming a semi-corrupted fusion.
It's sort of like if MC Bear-Bear didn't tear the fabric of his arm, but the fabric of his mind.
|—Garnet, "Monster Reunion"|
The appearance of the Corrupted Gems varies greatly with different Gems, although in "Monster Reunion" multiple Centipeetles (presumably Nephrites) can be seen, hinting at the possibility that like Gems will take on the same corrupted forms. This is also supported in "Earthlings", in which several corrupted Jaspers can be seen having taken wolf-like forms. As the bodies of Gems are mental projections, the bodies of corrupted Gems are a reflection of how damaged their minds are. Forms range from slightly humanoid to monstrous, and in severe cases, not having a physical body.
Corruption sometimes also changes the physical appearance of a Gem's gemstone. As Nephrite's gemstone had a curvy contour line that surrounded the circumference of the gem and upon partial healing by Steven, the curvy contour line straightened. Jasper's gemstone, however, wasn't changed by her corruption. Time may be a factor in this case - Nephrite had been corrupted for several millenia, as opposed to Jasper, who had just undergone it.
Most corrupted Gems are hostile and dangerous and behave similarly to wild animals. Many of them will exhibit behaviors such as feeding and predation, despite not requiring sustenance. However, much like regular Gems, when their bodies are severely damaged, they explode into smoke, or "poof", leaving their gemstone behind. The gemstone can then be safely contained in a Gem bubble, preventing it from regenerating. Some corrupted Gems are imprisoned in objects (such as the Pyramid Temple's gemstone), which also prevents them from regenerating.
Corruption also seems to strip gems of some of their basic powers. They are unable to summon their weapons or use warp pads and other Gem structures, such as a door to a spaceship.
Garnet simply describes corruption as a tear in the mind of a Gem, rather than physical damage on their gemstone and as such is harder to heal than a cracked Gem. There is possibly a way to reverse its effects, but it requires the power of all four Diamonds.
Steven's healing spit only partially and temporarily healed Nephrite, but she reverted to her corrupted form shortly after. Garnet acknowledges this as a major step in trying to heal the Corrupted Gems even though Steven's attempt had failed, saying that it was the best that they have seen it work. She indicated that Rose Quartz was unable to heal the Corrupted Gems to the same extent her son, Steven, had done.
In "Legs From Here to Homeworld", the combined powers of Steven, Yellow Diamond and Blue Diamond are able to heal Centipeetle back to Nephrite but the Corruption immediately takes back over when the three stop touching her. Yellow believes that the addition of White Diamond's powers would be able to permanently heal the Corrupted Gems.
Known Corrupted Gems
- Nephrite (Centipeetle Mother)
- Obelisk (Pyramid Temple Gem)
- Worm Monster
- Desert Glass 
- Big Bird
- Heaven Beetle
- Earth Beetle
- Gem Cave Creature
- Watermelon Tourmaline (Giant Pufferfish)
- Ice Monster
- Invisible Gem Monster
- Lighthouse Gem Monster
- Water Bear
- The Slinker
- Crab Gem Monster
- Unknown Giant Gem Monster
- Nephrite's Crew
- Snow Monster
- Biggs Jasper (Orange Great North Monster)
- Cobbled Beta Kindergarten Monster
- Green Striped Beta Kindergarten Monster
- Red Striped Beta Kindergarten Monster
- Purple Striped Beta Kindergarten Monster
- Spotted Beta Kindergarten Monster
- Beta Kindergarten Water Bear
- Corrupted Jasper Fusion
- Tongue Monster
- Flower Monster
Comic book series
- Obelisk (comics)
- Rainbow Cloud Monster
- Slug Monster
- Plant Monster
- Old Book Gem
- Invisible Manta Ray
- Clock Tower Gem
- Unknown Giant Bird
- The Perils of Pweepwee
- Unknown Bird Gem Monster
- Slime Gem Monster
- Unknown Lizard Gem Monster
- Red Eel Monster
- Monster Lizard Gem
- Mole Gem Monster
- Frozen Fragment
- Scorching Shard
Steven Universe: Too Cool for School
Steven Universe and the Crystal Gems
- When regenerating, Nephrite briefly assumed a humanoid shape – presumably reflective of her original form.
- It is unknown how many of the corrupted Gems are former Homeworld Gems and how many are former Crystal Gems.
- However, it can be assumed that the majority of corrupted gems were Crystal Gems, due to them not knowing about fleeing the earth.
- Some corrupted Gems, such as the Worm Monster and Nephrite (the Centipeetle Mother), are shown to have an urge to eat food. The reasons for this are unknown, but it is possible that they do not gain sustenance from their gemstones as most other Gems do.
- As shown by Nephrite and her crew, it is possible for Corrupted Gems to be peaceful and even retain some aspects of their former personality, as Nephrite and her surviving crewmates retained their bond with one another, despite becoming corrupted.
- As shown by the two Northern Gem Monsters from "Gem Hunt", Corrupted Gems are capable of feeling fear and their behavior indicates that Corrupted Gems may have a strong fight-or-flight instinct. This may suggest that Corrupted Gems are not innately violent or hostile, but instead possess a mentality similar to a wild animal, attacking and/or fleeing from perceived threats. This may also explain Nephrite's passive behavior towards Steven and the Crystal Gems after reuniting with her crew despite being corrupted, as she no longer saw them as a threat as she originally did in "Gem Glow".
- In "Monster Buddies" and "Crack the Whip", it is shown that Corrupted Gems can in a sense be tamed, though with varying results. In "Monster Buddies", Steven was able to befriend and bond with the regenerated Nephrite. In "Crack the Whip", Jasper managed to tame the two Corrupted Gems she captured in "Gem Hunt", through the use of intimidation and physical domination. While seemingly effective at first, due to her method's cruelty, the two Gems lack any real loyalty to Jasper as they followed her mainly out of fear of her and would abandon her if given a chance as shown in both "Crack the Whip" and "Earthlings".
- Unreleased corrupted Gems can be seen on Guy Davis' Tumblr, such as the Stinger, the Fist Chest, and the Shell Back.
- Being poofed or put into an object can save a Gem from being corrupted by the Corrupting Light. It is unknown whether being trapped in an object or being poofed in general is what saved Lapis.
- It is revealed in "Earthlings" that corrupted Gems are capable of fusing with regular Gems.
- However, this will cause the uncorrupted Gem to become corrupted once the fusion is ended (as shown by Jasper's corruption). It is currently unknown if Corrupted Gems are capable of fusing with other Corrupted Gems.
- It is implied that Gems of the same type have the same corrupted form, as seen with Jasper, Biggs Jasper, and the Snow Monster, both of the Water Bears, and Nephrite and her crew.