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The Crystal Caverns is an Undertale/Steven Universe crossover AU. It can be found here and is created by tumblr and reddit user RhymeBeat. The story takes place Underground with Steven Universe's Gems replacing monster characters. The underground was formed thousands of years ago as a fallout center for Rose Quartz's Crystal Gems, only to discover that the barrier protecting them, also trapped them all there.

The Crystal Caverns stars Rose Quartz's son, Steven Universe. He is half human and has lived the entirety of his life Underground in the Gem Ruins. He was raised by Pearl, an intelligent Gem who was devoted to his late mother. The story begins when he makes contact with a rosebed on the Ruins outskirts, connecting him with his "subconscious". Soon afterward, a talking rose by the name of Rosey teaches him about dodging, warning him that his home isn't as safe as he believes it is. Pearl comes to speak to him, revealing a series of puzzles that she set up to entertain him, all the while the Ruins proves to be more dangerous than Steven believes.

This danger, combined with Rosey's prompting leads him to leave the Gem Ruins, facing Pearl to prove that he is able to survive outside. His overall goal is to put a stop to the deaths of human visitors across the Underground, and to free the Gems in some other way. He has no idea how to do either of those things.

Most major characters in the story are Gems. Gems are alien sapient rocks that can project a humanoid holographic form from their gem to interact with the world. Gems are immortal beings that can only die by being "shattered". To shatter a Gem is to destroy the gem itself. All other severe injuries result in poofing. When poofed, a Gem will retreat into her Gem for the sake of reforming her physical form at a later time. Gems are referred to with feminine terms by default but they entirely lack a biological sex. Two willing Gems can undergo fusion. Fusion combines two Gems into a single being with a distinctive personality and powerset. Fusion requires continual mental synchronization or the component Gems will defuse back into their original states.

Gems come from a planet known simply as "Homeworld". On Homeworld, Gems are governed by a strict caste system that assigns Gems tasks according to the purpose they were designed for. Gems of the same type are largely identical in appearance and voice and are expected to have similar talents. On the top of this hierarchy are the Diamonds, powerful Gems that are considered flawless by those below them. Gems who come out deformed, or who fuse with Gems outside their Gem type are considered "Off Color" and marked to be shattered.

The Crystal Gem rebellion was created both to protect the Earth, and to allow Gems to live their lives free from the restrictions of Homeworld society.

Character Changes

Steven Universe (Role: The Protagonist)

  • The protagonist of the story. A human-Gem hybrid and the son of Rose Quartz and Greg Universe. He is shown talking both in and out of combat.
  • He has two interconnected souls, his own pink human soul and his mother's Gem. In the vast majority of cases, enemy attacks aim for the former.
  • He is a kindhearted child, who is quick to believe in other people. He refuses to give up on the Gem Monsters that share his home with him, even though they do not remember his actions, or even the names he gives them.
  • He is able to use his Gem to summon a magic bubble to block attacks. He can also grow plants. His powers slowly develop over the course of the story.
  • He likes bad puns and is quick to laugh, assuming that the joke is not at the expense of someone else.
  • He is a good listener and often will spend time to understand the root of a Gem's problems before trying to solve them.
  • He has a talent and enjoyment of music, his late father was a musician and Pearl taught him to play a variety of instruments due to her own love of music.
  • He gradually grows to understand Pearl as his mother figure, but he is frustrated by her unwillingness (and inability) to be fully honest with him.
  • His Gem chemistry is abnormal for his Gem type, consisting entirely of Carbon.
  • When Steven is angry, he can access his super strength. With it, he's capable of shattering a large (albeit cracked) boulder into pieces.
  • Steven discovered his ability to save and reset after he died at the hands of Fused Quartz. He was upset and confused at this revelation, but refused to embrace violence despite it being the easier way to handle her.
  • He can read and speak Gem due to living his life Underground. He also has the unique ability to understand Corrupted Gems through their distorted vocalizations.
  • Despite his frustration with Leggy's constant tests, he still steps in to save Leggy from a corrupted Gem without barely a second thought.
  • His background in the Ruins has left him with virtually no real friends.
  • He wishes he was able to have a relationship with his parents, even understanding that they're both dead.
  • Steven is excited to explore a Gem settlement and learn more about Gems due to his isolated upbringing.

Connie Maheswaran (Role: The Fallen)

  • Initially posing as his "subconscious" Connie is the narrator of the story. She knows quite a lot of useful information about Gems and will help Steven with solving puzzles if the solution is particularly obstuse.
  • She refuses to explain why she's in his head, claiming it's "embarassing".
  • She knew both Pearl and Greg when she was alive.
  • Connie is more logically minded as opposed to Steven who favors emotional responses most of the time.
  • She is willing to listen to Steven deal with his problems, and she herself reveals some of her background during these conversations.
  • She discovered Determination before her final death via dying during swords practice. She refuses to tell Steven why her final death ended up as permanent.
  • She was trained by Pearl and thus can lend Steven her experience with the sword.

Rosey (Role: The Empty One)

  • A magic talking flower that initially poses as Steven's plant guardian, an artificial being that was created to protect her master.
  • She reveals herself to know much more than a creature in her position should know, clearly having some insights into Greg, who would have died before she was created.
  • She helps Steven after Pearl leaves, all the while snarking at him and his happy-go-lucky methods of problem solving.
  • She's often the one to suggest the more pragmatic decision, whether it's leaving an injured Gem trapped, or poofing an enemy proven to be a larger threat.
  • She is more powerful than she appears, being able to poof a large Gem Monster quickly and easily, and able to block off a path with florakinesis that overpowers Steven's own.
  • In reality she hates spending time with Steven, as she finds his naivete and endless positivity insufferable. Despite this she never actually harms him and wants him to successfully journey across the Underground,

Pearl (Role: The Caretaker)

  • Steven's adoptive mother and Rose's closest confidant.
  • She is highly intelligent, able to design and engineer several puzzles to keep Steven entertained.
  • She constantly compares Steven to his mother, believing that he has the potential to become as great as she is over time.
  • She wants to preserve the history of the Gem Ruins, while lamenting that the area is currently virtually uninhabitable due to the Gem monster infestation.
  • She can summon Holo-Pearls. Simple automatons that are a holographic versions of herself. The Holo-Pearls are single-mindedly devoted to fighting and training.
  • She likes baking but refuses to eat herself. She likewise never sleeps, but uses her time to do other things.
  • She hides several things from Steven, such as the true fate of Greg and the existence of an exit to the Ruins that Steven can use.
  • Pearl is a Gem stuck in the past, having only Steven to bind her to the present. She cherishes everything about Rose Quartz and considers herself a failure for not being able to fulfill the promise she made to Rose.
  • She continues to accept calls from Steven after he leaves the Ruins, giving him some information on topics he's not entirely familiar with.
  • Pearls are designed to act as servants for high status Gems. Pearl has far surpassed her original function by learning how to fight and think,
  • Pearl is incapable of talking about either her former owner, or an incident referred to historically as "The Great Shattering".

Lapis Lazuli (Role: The Recluse)

  • She was trapped in a mirror that had been buried in the sands of the Gem Ruins. Steven accidentally stepped on her Gem when traversing the area where she was buried, a normally crippling injury for a Gem.
  • She lashed out at Steven for this injury before calming down and realizing that her injury was an accident.
  • Previously she had been trapped in the same mirror, only to be released after the barrier's foundation.
  • She was attacked by behind 12 years ago as a part of a plot against the Queen of the Underground.
  • She is designed to be a terraformer Gem, and thus has the power to move large amounts of water around quickly and powerfully.
  • She worked at the post office before her disappearance, leaving behind friends who are worried about her safety.
  • She was healed via unknown means, but after being healed she was able to flee from the Ruins by flying out of the holes in the ceiling.
  • She likes to joke around when she's feeling calm.

Padparadscha (Role: The Judge)

  • She is a Sapphire Gem. Sapphire Gems usually have the ability to see the future. However she came out defective and is instead "gifted" with the ability to see the past, which invariably leads to predictions of things that have already happened.
  • She maintains the Boreal Forest area, using her plant powers to keep the trees alive without sunlight, and fixing any machines that are broken or in a state of disrepair.
  • She was asked by Pearl to protect Steven, a task she immediately accepted due to its importance.
  • She is friends with Leggy, a Ruby who wants to capture a human. Despite her friendship she is still dedicated to prove that Steven is a Gem and not a human.
  • She is jealous of normal Sapphires, lamenting her inability to see the future.
  • She has an exceptionally good memory and a knowledge of history, chemistry, and mechanics.
  • She was previously friends with a human called Lars who passed away some time ago.
  • Her past vision was previously uncontrollable, leaving her permanently stuck a few minutes behind everyone else. In situations of high stress she reverts to this state.
  • She's a member of the Off Colors, a group of outcast Gems that have left Homeworld behind for Earth.
  • She met Leggy after she moved Underground, and Leggy encouraged her to try to be better than she was. This was the beginning of their friendship.
  • She learned to control her past vision by shifting her visions to see a few seconds into the past.
  • Her past vision makes it difficult for her to fuse, as even her workaround doesn't allow for perfect synchronization with her partner.
  • Leggy accuses her of projecting her feelings about the past on Steven. Despite this Padparadscha sees her protection of Steven as how she moves on from the past.
  • Padparadscha can hover over the ground if need be.
  • Padparadscha is terrified of Biggs Jasper and considers her a dangerous beast.
  • Padparadscha visits Lars's grave frequently and grows flowers on it.
  • Padparadscha took Lars's death hard due to a complete lack of experience with death.
  • Live on Homeworld was ruled by fear of being shattered for being different for her and the other Off Colors.
  • Paddy was set on the path to manage her past vision by a bet among the other Off Colors over who could fix their crippling flaws the soonest.
  • She is the only one truly capable of reaching through to Leggy.
  • She considers Steven among her friends, and encourages Leggy to see him the same way.

Leggy (Role: The Ambitious One)

  • Leggy is a Ruby, a common soldier Gem that is considered dull and expendable.
  • Leggy has a strong desire to join the Royal Guard, but is willing to wait till she has proof of Steven's humanity before attacking Steven.
  • Leggy writes off Steven's powers as some kind of Jedi-esque "Force" ability. She heard stories similar to Star Wars from Lars and is unable to distinguish those stories from the realities of life among humans.
  • Despite his name being used several times, she always refers to Steven as "human".
  • Leggy is quite insecure, believing herself to be bad at the one thing she was designed to do, fight. Her summoned Gem weapons are caltrops which she perceives as equally useless.
  • She actually knows a fair bit about humans and their needs and has done research on how human souls work.
  • She is an artistic individual who has a particular interest in fashion design. Gems do not need physical clothes, but several Gems are impressed by her handiwork and wear them regardless. However she has recently given up many of her artistic endeavors to focus on her Royal Guard training.
  • She was trained by Tiger's Eye in the art of combat. Tiger's Eye encourages her to fulfill her purpose as a warrior, despite her possessing other talents.
  • She can be very dedicated to executing the plans she comes up with. But she occasionally puts lots of effort into a plan without first evaluating the plan on its own merits.
  • Leggy wants to compromise with the Royal Guard, allowing Steven to live out the rest of his natural life Underground. Padparadscha questions the other Guard's willingness to do this.
  • Leggy sees protecting Steven as harmful to Padparadscha, believing she's projecting her past relationship onto him and attaching herself to someone who will inevitably die.
  • Leggy has a poor memory, but tends to forget this particular limitation.
  • Leggy, alongside all other Rubies, has the ability to summon fire based attacks.
  • She considered herself "just like everyone else (the Rubies) but worse". This led to her always following behind the others in all decisions.
  • Leggy was sent to Earth to fight the Crystal Gems.
  • At some point Leggy was on a baseball team with the other Rubies.
  • Training from Citrine, Tiger's Eye, and Amethyst has given her a stronger sense of individuality and competence.
  • She is aware of her artistic talents, but refuses to give in on her dreams when given the chance to set them aside.
  • She admires Tiger's Eye but was furious when she discovered that she was being used to capture Steven outside the bounds of the law.
  • Despite claiming her ability to do anything, Leggy's confidence in her own abilities is a front she puts up for others.
  • Leggy was close to Lars Barriga when he was alive, and learned a lot about art and sewing from him. After his death she gave both of those things up and decided to rededicate herself to being the best at fighting.
  • Leggy's trauma over Lars's death has also made her hesitant to befriend another human.
  • Despite all of her hangups, Leggy puts Padparadscha's happiness above her fears and accepts Steven's friendship.

Additional Changes

The Crystal Caverns largely uses custom tilesets and does not reuse locations from the original Undertale. The Gem Ruins in particular are based on various examples of Gem architecture and the beachlike environment surrounding Steven's home in Beach City.

Every monster, NPC and plot critical character is replaced by a Gem, including Gems original to TCC. All humans are replaced by human characters from the show, save for Steven himself who is half-Gem.

Alongside the Fight, Act, Item, and Mercy buttons is a Gem button. This allows Steven to use defensive powers or to use his abilities to achieve actions not possible for a normal human being. Using his Gem powers cost PP (Power Points) and cannot be used when his PP hits zero.

The Cell option is replaced with the "Comm" option, standing for communicator. The communicator uses the shop interface, showing the Gem on the other side of the line in full color in a detailed environment. Steven can also access his Gem powers on the overworld via the Gem option in the menu.

Steven can gain EMP alongside EXP and LV. EMP is gained by sparing Gems through the appropriate Acts. EMP stands for Empathy and allows Steven to upgrade his powers and unlock new abilities.

Unlike most protagonists, Steven was born Underground and has never known the surface.

Story