Unmarked spoilers ahead.
Confirmed in Season 1
- Many people correctly guessed that the girl from the title sequence (who was later revealed to be named Connie) would be Steven's love interest.
- Some viewers were correct in guessing that the monsters the Crystal Gems fight are other Gems, which was hinted at in "Monster Buddies" and later confirmed in "Ocean Gem".
- It was guessed pretty early on by observant viewers that Garnet was likely a fusion due to having two gemstones, with some even narrowing it down to her being composed of a Sapphire and Ruby based on the colors. This was confirmed in "Jail Break".
Confirmed in Season 2
- Lots of people theorized that Sardonyx would be the fusion of Garnet and Pearl, which was confirmed at a Comic-Con panel in 2015, before being showcased in the show itself in "Cry for Help".
- A lot of people correctly guessed that Sapphire has only one eye, which was officially confirmed in "Keystone Motel".
- Vidalia being Onion and Sour Cream's mom, and Yellowtail's wife, confirmed in "Onion Friend".
- Peridot's body being artificial in some way, while not as widespread as some other theories on this list, was still pretty popular. Although many took this all the way and asserted that she was merely a Gem controlling a robot body, prior to her detaching her foot in "Friend Ship", at which point many shifted toward believing that she had prosthetic limbs, which turned out to be the case in "Catch and Release". A decent amount of fanart even got very close to guessing how she looked without the robotic limbs.
- "Back to the Barn" confirmed that there are not only multiple Pearls (and that they are a Servant Race on Homeworld, as many assumed) but that there are multiples of every Gem. That said, many got the function of Pearls wrong, believing their niche to be smart and tech-savvy assistants, with our Pearl's gracefulness being something she picked up on Earth. It's really the other way around, with Pearls being singing and dancing entertainment at best.
- "The Answer" confirmed, among other things, the existence of multiple Diamonds, who seem to be in a position of royalty, something theorized since "The Return".
- Quite a few people had actually correctly predicted that Peridot's HeelFace Turn would end with her growing disillusioned with Homeworld after establishing contact with Yellow Diamond. "Message Received", would have Peridot unsuccessfully try to convince Yellow Diamond of the planet's strategic value, only for Yellow Diamond to state that she didn't care about that; she just wanted the planet destroyed out of spite (which itself confirmed a theory had about the creation of the Cluster).A few even predicted she'd call Yellow Diamond a "clod" in the progress.
Confirmed in Season 3
- Many people correctly guessed that the Watermelon Stevens would be involved in some way with the return of Malachite. "Super Watermelon Island" revealed the Zerg Rush of the former hindering the latter so that Alexandrite could finally win the match.
- Many people guessed that Lapis was a civilian who wasn't an active participant in the war, and who just got caught in the crossfire while doing her usual work.
- A humorous example of this trope came in someone correctly guessing that Lapis' human name in "Hit the Diamond" would be "Bob".
- Many fans predicted that "Drop Beat Dad" would involve Sour Cream and Marty, and that it would confirm the latter as the former's biological father.
- By this point, most fans assumed that Gem Corruption was caused by the Diamonds. "Monster Reunion" would confirm it.
- Many fans correctly predicted that Jasper would return in "Alone at Sea".
- About a year before the airing of "Earthlings" and the debut of Steven and Amethyst's Fusion, the Tumblr blog Texts Between Gems joked that such a Fusion would have a yo-yo as a weapon.
- The existence of a Pink Diamond. Many fans had predicted the character's existence since Season 1, long before the show would confirm the existence of a Diamond Authority in general, thanks to various Gem structures having a symbol of four diamonds forming a bigger diamond, instead of three intersecting triangles, with the latter missing the color pink. It was though that she was the ruler of the Earth colony, the rebellion was against her, and that she was killed or otherwise disappeared during the Great Gem War. The last three episodes of the season would confirm all these things.
- Fans additionally guessed that the symbol placement corresponded to Gem placement on the body: Pink had it in her navel, Yellow and Blue on the chest, and White on the forehead. Each character's eventual appearance throughout the other seasons would confirm this as well.
Confirmed in Season 4
- Due to the Crystal Gems not being documented citizens, them not understanding many human traditions, and Greg being open-minded about many things, many fans speculated that Steven was conceived out of wedlock. "Gem Harvest" confirmed that Greg and Rose never married each other.
Confirmed in Season 5
- A lot of people of theorized that Dewey would start working at the Big Donut following his electoral defeat in "Dewey Wins". Sure enough, come "Letters to Lars", this happens.
- After lots of red herrings and general all-around mystery, one of the most popular fan theories since at least "Sworn to the Sword" came true in "A Single Pale Rose": Rose Quartz really is Pink Diamond. She shapeshifted into Rose Quartz to lead a rebellion against herself to save the planet that she'd grown to love but couldn't officially spare, and faked her own shattering in a bid to get Homeworld to leave, assuming her Rose Quartz identity permanently. This is one of the show's most interesting examples, as many fans who did subscribe to this theory abandoned it at the end of Season 3, when the show had revealed that Rose killed Pink. A "fact" that every Gem on the show believed (or, in Pearl's case, perpetuated) until the plausibility of it was questioned in-show at the beginning of Season 5. This Tumblr post summarizes many of the things fans picked up on.
- After Pink Diamond's gem is shown to have an upside-down brilliant cut in "Jungle Moon", people started theorizing that Rose's gem was Pink's, shapeshifted so that the pavilion pointed inwards instead of upwards. "A Single Pale Rose" reveals that that this is exactly the case.
- Around the time of "The Trial" to "Your Mother and Mine" (which is when we first see a flashback of the event), people started theorizing that the Gem eyewitnesses saw shattering Pink Diamond was Pearl, shapeshifted to look like Rose Quartz (the trauma related to the event being why she hasn't shapeshifted since). This was confirmed to be true in "A Single Pale Rose" though Pink Diamond was only apparently shattered, but actually only poofed.
- And one last theory confirmed in that episode was that every time Pearl covered her mouth before she said something regarding her past with Rose was not her trying not to say something Steven was not ready to hear, but rather a literal involuntary reaction to say something she literally cannot talk about. Sure enough, it is the remnant of Pink Diamond's last order to her as a Diamond, that they were to never speak of the event again.
- Following some leaked keychains showing Ruby and Sapphire in wedding attire, fans started speculating that the two would get married, which they do in "Reunited".
- Quite a few people predicted that Centipeetle and her crew were Nephrites, and her commander being a Hessonite, based on her drawing in "Monster Reunion" resembling the Hessonite in Save The Light.
- Even before the title of "Legs From Here to Homeworld" was leaked by CN France, giving it away in a sense, fans correctly guessed that Pink's ship would be shaped as legs, and would be the pink structures seen in front of Rose's landfill, seeing as Yellow Diamond's ship was a right arm, Blue's was a left arm, and a structure showing White Diamond's head and torso is seen in "The Trial", confirmed in "Legs From Here to Homeworld" to be her ship. "Change Your Mind" confirms that the ships make up a Combining Mecha.
- Steven's dream at the beginning of "Together Alone" confirms a theory about White Pearl that has been popular ever since her debut: that she used to belong to Pink Diamond, based on their shared gem placement as also seen with Yellow and Blue's Pearls.
- Many predicted that Blue Diamond would be the first of the Diamonds to change sides, since she was the only one of the Diamonds willing to make amends. "Change Your Mind" confirmed this to be the case, after she realizes how terrible Pink Diamond felt under the pressure she and the other Diamonds have put her through, going even so far as to directly rebel against Yellow to help Steven.
- Many people (some jokingly) predicted that the fusion of Steven and Pearl would have an umbrella as a weapon. The fusion also calls themselves "Rainbow 2.0", which is likely taken from the fandom.
- Many people predicted that the fusion of all four Crystal Gems is named Obsidian, based on early concept art of the Crystal Temple in Art & Origins calling it the "Obsidian Temple".
- Even the darker twists in "Change Your Mind" were predicted: this Tumblr comic, originally published on July 29th, 2018 (about 6 months before the episode's premiere) and this animation from October 5, 2018 both predicted White Diamond pulling out Steven's gem, but with a significantly different outcome than what would happen in the show, and this edited screenshot on Reddit predicted the Crystal Gem Pearl suffering the same fate as White Pearl.
- Speaking of White Pearl, from the moment she made her brief but memorable debut appearance, almost all fanfics involving White Diamond worked on the assumption that (a) White Pearl's strange appearance and behavior was the result of something White Diamond did to her, and (b) whatever she did to make her that way is something she can use on other Gems to turn them into extensions of herself, which alone would make White Diamond a truly nightmarish foe. "Change Your Mind" reveals that the behavior was because of White Diamond's puppeteering her, though Future would reveal in the episode "Volleyball" that her appearance was because of Pink.
- Following her corruption in "Earthlings", many comics featuring a healed Jasper depicted her as carrying some of her "monster" traits over. Usual choices were teal spots, horns, and maybe a spike or two. Come "Change Your Mind", the ending shows Jasper returning with spots and horns, as well as other Crystal Gems and Homeworld Gems saved from corruption.
Confirmed in Future
- The theme song for Steven Universe: Future showed a winged fusion that many fans predicted would be a fusion between Eyeball and Aquamarine because of the shape of her wings, her colors being red and blue, her hairstyle looking like a combination of the two, and her glowing teardrop-shaped gemstone. Sure enough, this theory was confirmed correct when a promo that was leaked on November 18th, 2019, showed their gems in a close-up shot.
- Many have predicted that Pearl would be the one to help Pink Pearl deal with her trauma, in "Volleyball" exactly this happens.
- A lot of people correctly guessed that the monster seen in the opening for Future was actually Steven himself, due to the fact that it was the only one to be moving in the shot, and its nose looking a lot like his. The fact that over the course of the series, Steven's mental state deteriorated only made the theory more popular, believing he would self-corrupt. While it's left unclear whether it really was corruption or something else, he did indeed turn into the monster in the four-part finale.