First, you need a gem at the core of your being. Then you need a body that can turn into light. Then you need a partner who you trust with that light.
"Fusion Gems" are the product of two or more Gems (or, in a special case, a half-Gem, like Steven Universe, and a human, such as Connie Maheswaran) who fuse together, creating an entirely separate entity. Fusions are commonly formed in states of emergencies and used as a last resort, except in cases like Garnet, where Ruby and Sapphire decided to stay fused forever as a sign of their love.
When Gems fuse, both their bodies and minds become one. This creates an entirely different entity, as opposed to two minds sharing a single body. Some dominant features in Gems can occur in their fusions, such as Pearl's nose shape. Malachite, Garnet, and Alexandrite, showed that one component Gem can take control of the fusion. It is also possible for different Gems to take control of different parts of the fusion, as proven by Lapis Lazuli's control of Malachite's hydrokinesis while Jasper was in control of the body. For Malachite and Alexandrite, the Gem in control's voice was layered over the fusion's voice to show control. However, for Garnet, this was not done; it was only the fusion's voice, although it is unknown whether this is intentional. As seen with Sugilite and Malachite, certain fusions will cause the loss of individuality between their constituent Gems if fused for long periods of time. While all fusions require consent, a fusion of hatred is inherently unstable.
Memories made before and during a fusion can be held over, as Opal, Sapphire, and Sardonyx recognized Steven despite him never meeting them personally. However, Gems who fuse together don't automatically share their memories with each other. For example, the second time that Garnet and Pearl fused into Sardonyx in "Cry for Help", she did not know that Pearl had secretly repaired the tower.
In order to fuse, Gems must to be in perfect sync mentally, physically and emotionally. Fusions are formed when Gems perform a synchronized dance. Each fusion's dance is different. If successful, Gems transform into an amorphous mass of energy before fully transforming. During the process of fusion, the gemstones of the participating Gems are visible. However, Sardonyx was the first fusion Gem not to show Gemstones during fusion. If unsuccessful, Gems will be ejected before finishing the entire transformation. An example of a stable fusion (and de-fusion) was shown in "We Need to Talk", when Rose and Pearl fused into Rainbow Quartz. Garnet gives Greg a more literal explanation that Gems turn into light and have to be trusting of the other Gem(s) with their light. During Fusion, each gemstone from the fusing Gems are visible and seen shifting into place while the mass forms into a body. However, in "Cry for Help", when Pearl connected with Garnet, both Gems and gemstones disappeared before the mass of energy appears. Even when Sardonyx is fusing, the gemstones are not visible until Sardonyx fully fuses.
Fusion order does not matter for multi-Gem fusions. The same Gems will produce the same fusion regardless of which order they combine in. For example, if Opal fused with Garnet, or if Pearl fused with Sugilite, or if Amethyst fused with Sardonyx, each resulting fusion would still result in the formation of Alexandrite.
Each fusion embodies a relationship between the participants. While some fusions like Garnet embody romantic relationships, fusion is not inherently romantic, and other fusions can embody other sorts of relationships such as the social relationships between friends, allies or even enemies.
De-fusion is a process in which a fusion splits into its component Gems. During the de-fusion process, fusion Gems will reverse the fusion process. The fusion will turn back into a mass of light before splitting back into its original participating fusees. During the de-fusion, the Gemstones of the participating Gems are visible at all times. However, in "Cry for Help", Sardonyx was the first fusion not to have Gemstones during de-fusion.
A stable de-fusion occurs when the participating fusees have a mutual agreement on de-fusing. This is shown in "We Need to Talk" when Pearl and Rose Quartz de-fuse from Rainbow Quartz, they smoothly shift out of each other. Another example of a stable de-fusion is in "Cry for Help" when Sardonyx de-fuses into a flash of light, leaving Garnet and Pearl holding each other.
An unstable de-fusion occurs when the fusees have a very high conflict of opinion. This is shown by the fusees being thrown out of the fusion. The first example of this is shown in the episode "Giant Woman" when Pearl and Amethyst argue over the Heaven Beetle, causing the fusion to become un-synchronized forcing them to de-fuse. This is also with Alexandrite arguing over a bit of shrimp, causing de-synchronization and un-fusion, and with Garnet defusing caused by Sapphire and Ruby's conflicted feelings due to Pearl's deception. Similar incidents nearly occurred with Malachite as she was being pulled to the bottom of the sea and Garnet when she was faced by a Cluster Gem.
Another unstable de-fusion through conflict was shown in "Keystone Motel". When Garnet's component Gems are arguing over Pearl's betrayal of their trust, Ruby ends the fusion by speaking through Garnet 'Then you can just go!' ending the fusion, and ejecting the fusing Gems.
A damaged de-fusion occurs when the fusion takes on severe physical damage, as shown in "Coach Steven". When Sugilite is injured by her flail, she 'poofs', back to Garnet and Amethyst. During a damaged de-fusion, the original Gems do not retreat into their gemstones to regenerate, however the damage caused to the fusion will be carried on to the component gems as Garnet and Amethyst were unable to move and had 'a killer head-ache'. Unless they are dealt in contact with a great amount of physical damage, or a Gem Destabilizer, the fusing gems won't retreat into their gemstones to compensate for the damage to the fusion.
An example of this is when Jasper stabs Garnet with a Gem Destabilizer, causing Garnet's body to fall apart. She poofs back into her original Gems, Ruby and Sapphire. However, unlike Sugilite, the damage caused to Garnet's body also caused severe damage to both Ruby and Sapphire, forcing them into their gemstones to regenerate. It is unknown how the Gem Destabilizer is capable of doing this, or how it is engineered.
The new forms have extra body parts, and the combined heights and strengths of the fusers.
|—Garnet, Fusion (short)|
Fusion Gems combine the designs of both/all Gems unless they are the same type of Gem, then they'll just be a bigger version of their Gem design, as shown in "The Answer". Gemstones appear in the same locations and are the same cut of the Gems are composed of, but the type of Gem changes. The exception would be Stevonnie, whose single Gem remained as Steven's. Should two Gems with gemstones in the same location fuse, the gemstones would be side by side on the resulting fusion. Each Gem involved in the fusion often contributes an additional pair of arms, showing that multiple pairs of arms are a common trait among fusions. Another common trait among fusions is having more than two eyes. Many fusions so far have had as many eyes as all of their constituent Gems put together. Opal, Stevonnie, some of the Cluster Gems, Sardonyx, and the Ruby fusion have had fewer. Alexandrite's eyes have yet to be seen, since they were covered by a visor during her appearance. Fusions also base their appearance on what they think would 'Look cool' suggesting both fusing gems have a choice on which attributes they can contribute, disproving that the Fusion's form does not represent their stability.
Types of Fusions
♫ Can't you see that my relationship is stable? ♫
|—Garnet, Jail Break|
A fusion is marked stable when the fusing gems remain a strong synchronization, while also keeping a strong bond between each other. Some fusions are stable due to how close in connection the Gems fusing are, such as Garnet, due to the fact that Ruby and Sapphire are so romantically involved.
Garnet, think about this. You and Amethyst can be a little... uh... unstable when your personalities combine.
|—Pearl, Coach Steven|
Some fusions can be unstable due to the way their participants' personalities combine. In "Coach Steven", Garnet and Amethyst lose themselves in the Sugilite fusion, going on a rampage and refusing to split up. It is assumed that both Ruby and Amethyst were the dominant personalities in Sugilite, overshadowing Sapphire's; because of their desire to stay fused, Sugilite began to lose her rationality.
We're Malachite now...
|—Lapis Lazuli, "Chille Tid"|
While most unstable fusions fail upon fusing or during the fusion, Malachite is the only example of an unstable fusion remaining fused. This is due to the pure will-power of Lapis Lazuli, as we see in "Chille Tid" she is holding the fusion together with all her strength, while fighting Jasper to keep the fusion together. However it is unknown if both the fusing gems have lost their sanity and now truly identify as Malachite.
She had control when she used her power to drag their fusion into the ocean. But a fusion like theirs is unstable, bound together by anger and mistrust. If that bond snaps, their anger will take over, and destroy.
|—Garnet, Chille Tid|
In addition to the usual co-operative fusions, it is also sometimes possible for two opposing Gems to successfully fuse with each other, and then struggle for control without defusing. However, this type of fusion is especially unstable, and fusion participants run the risk of becoming overwhelmed by their shared anger and losing control.
Malachite is the only known example of a successful hostile fusion. In this case, both participants had similar motivations but conflicting goals (Jasper wanted revenge against Garnet and the Crystal Gems after her previous defeat; Lapis Lazuli wanted revenge against Jasper for imprisoning her and to protect Steven).
They were forced together! They were forced to fuse! This is wrong!
|—Garnet, Keeping it Together|
Through being buried underground and fusing due to extensive pressure, it is possible to force Gemstones or Gem Shards into fusing. This method of fusion is usually incomplete, simply binding Gemstones together on the edges instead of changing their colors, as shown in complete fusion. The Cluster Gems are a result of a forced fusion, and in "Keeping it Together", it was shown that Gem Shards could be forced to fuse. The physical bodies of these Gems usually embody the incomplete fusions, with limbs being stuck together at seemingly random places. More forced fusions examples were later shown in "Nightmare Hospital". In "When It Rains", Peridot referred to the process that created them as "artificial fusion".
It's never like that! Whenever I've fused, it's always just been me, but bigger, I... I've never had a third eye before.
|—Ruby, The Answer|
Whereas different Gems fusing creates a new Gem with a new personality, same-Gem fusions simply create a larger version of the Gems that are fusing. Because of the lack of individuality of mind and physical construct, there are no new personalities and all the Gems taking part in the fusion still feel themselves.
If two Gems are of the same type but are not identical, they would still form a larger version of themselves in build, but with a combined color palette and outfit. 
The hostile Malachite fusion has an internal fusion realm where Lapis Lazuli and Jasper struggle for control. It is not known whether this is a general property of fusions, an unusual property of Jasper and Lapis Lazuli's hostile fusion, or Steven's perspective on Malachite as viewed in his dream.
Another instance of a similar concept is when Garnet seems to converse separately as Ruby and Sapphire before or after starting to de-fuse in "Keeping it Together" and "Keystone Motel", giving light to their opposing opinions.
- Fusions are frequently pictured, especially as statues, throughout the abandoned Gem monuments in the show, (e.g. The Lunar Sea Spire).
- The Crystal Temple has the appearance of a typical fusion. The Gem placements suggest that the temple is a fusion of Rose, Amethyst, Garnet, and Pearl.
- In "Jail Break", Jasper derides fusion as a "cheap tactic to make weak Gems stronger", while Garnet states that she is a symbol of love.
- Garnet refers to Earth as 'The place where I'm free, to live together and exist as me', implying that Homeworld denied her the freedom of remaining fused as an embodiment of Ruby and Sapphire's love.
- Pearl implies fusion is rather intimate in "Coach Steven" and "Alone Together"; she tries to cover Steven's eyes while Garnet and Amethyst perform their dance, and says Stevonnie might be considered "inappropriate". In "We Need to Talk", Pearl says fusion is the ultimate form of bond between Gems. Garnet has also stated in "Keeping it Together" that fusion is considered sacred and to her, it is a choice.
- According to "Guide to the Crystal Gems", when Ruby and Sapphire first fused with each other, it was unusual for Gems of different types to fuse with each other. This is because Gems only fused with other Gems of the same type (rubies with other rubies and so on).
- On the Gem Homeworld, fusions are primarily used only in battles and as weapons. Being fused outside of a fight is seen as disgraceful, as Peridot is constantly uncomfortable around Garnet, calling her a perma-fusion and a filthy war machine.
- Fusion between Gems of different types is also seen as "digusting" and reprehensible by Homeworld Gems in "The Answer".
The abilities of Gem fusions vary based on the two Gems who fuse. Whenever a Gem fuses to make a weapon, they combine their own original weapons to create a new one composed of those two or more weapons. If a Gem possesses significant other abilities, such as Sapphire's future vision, that may also be incorporated into the fusion's abilities.
Fusions may wield the weapons of the Gems that compose them, but can combine them into a singular weapon. These weapons typically harness the abilities of the weapons that were fused, an example being Opal's weapon, a longbow. Pearl's spear not only makes up the longbow, but the arrows are visually similar to the energy blast the spear can emit, and the spear itself. Meanwhile, Amethyst's whip acts as the bowstring. Sugilite's flail uses Garnet's gauntlets as a mace head and Amethyst's whip as a rope. Sardonyx's war hammer is composed of Pearl's spear as the handle and Garnets's gauntlets as the head.
Fusions (by Gem number)
- Rainbow Quartz
- Unknown Fusion Gem
- 2 Unknown Crystal Gems
- Excluding same-gem fusions, only ten fusions are currently known: Garnet, Opal, Sugilite, Alexandrite, Stevonnie, Malachite, Cluster Gems as a whole, Rainbow Quartz, Sardonyx and Smoky Quartz
- Stevonnie and Smoky Quartz are the only known hybrid-fusions.
- Fusions with more participants appear to be larger and more powerful. Garnet, a two Gem fusion, is only seven feet tall, while Alexandrite easily towers over all other known fusions. However, the more the Gems, the more the risk they run of losing their individualities.
- It appears that the more hostile the fusion, the larger and more monstrous it becomes. Malachite, an unstable two Gem fusion, was formed and held together by anger and mistrusts and easily towers over the stable Garnet, who reaches up to a fraction of her forearm. Sugilite, a three Gem fusion, was easily large enough and powerful enough to destroy the rebuilt Communication Hub with a single punch.
- The statue forming the Crystal Temple appears to be a fusion of the Crystal Gems and Rose Quartz since it shows their respective gemstones, and has the most of any fusion shown thus far.
- Left palm/Ruby
- Right palm/Sapphire
- Navel/Rose Quartz
- It is the fusion depicted with the most arms, though some are missing due to the temple's age or the damage done to it (the Centipeetle Mother in Gem Glow destroys a hand with its acid spray). However, in all episodes except for the Pilot, which is non-canon, Rose Quartz's gem has not been shown due to its location being either the entrance to the Beach House or the Crystal Temple itself.
- The "Fusion Dance" displayed by Gems, is very similar to the Fusion Dance shown in the anime, "Dragon Ball Z". However, the Gems actually dance to fuse, rather than complete a set of poses like the Fusion Dance in Dragon Ball Z, in order to become one.
- Stevonnie seems to be an odd exception, since they separated while they were dancing in an attempt to embarrass Kevin.
- This could mean that fusions can separate when both or one of the components to the fusion become emotionally overwhelmed/compromised.
- Also, Garnet almost split apart one time for the same reason.
- Stevonnie seems to be an odd exception, since they separated while they were dancing in an attempt to embarrass Kevin.
- It is unknown whether or not Stevonnie, Malachite, Alexandrite, Rainbow Quartz, and Cluster Gems have signature weapons.
- Since Connie is not a Gem, thus having no weapon, it is assumed that Stevonnie's weapon would be Steven's shield.
- According to Garnet in the Fusion short, Fusion Gems have extra body parts. This is true for all Fusion Gems that are composed of different and non-hybrid Gems; Garnet has an extra eye, Opal has two extra arms, Rainbow Quartz has two extra eyes, Sardonyx also has two extra arms, Malachite has two extra arms that act as feet, Sugilite has two extra arms and three extra eyes, and Alexandrite has four extra arms, and an extra mouth, as well as likely having extra eyes under her visor. The Ruby Fusion and Stevonnie do not have extra body parts.
- Alexandrite is the only fusion whose dance wasn't shown
- However, part of her dance is glimpsed in Attack the Light.
- A brief fusion dance is later seen in the Classroom Gems Fusion short.
- Steven is capable of fusing with humans as shown in "Alone Together", although only Steven can do this as he is half-human.
- With the exception of Malachite and Alexandrite, all fusions are voiced by musicians, Garnet by Estelle, Opal by Aimee Mann, Sugilite by Nicki Minaj, Stevonnie by AJ Michalka, and Sardonyx by Alexia Khadime.
- It is unknown if Rainbow Quartz is voiced by a musician or not as she has currently not been heard talking yet.
- Garnet, Stevonnie, Rainbow Quartz, and Sardonyx are the only known stable fusions.
- Fusions tend to combine the complexions of the Gems that form them. However, there are exceptions; for example, Alexandrite is redder than Garnet or Amethyst, Rainbow Quartz is darker and more purple than Rose Quartz or Pearl, and Sardonyx is redder than Garnet.
- Fusions with more Gems tend to be larger. Sugilite and Alexandrite are larger than any known two-Gem fusions.
- Malachite seems to be an exception. This may be due to the size of Jasper, the hostility between Jasper and Lapis and the stability of their fusion overall.
- More stable fusions like Garnet and Stevonnie tend to be more compact and humanoid, while more volatile fusions like Sugilite, Alexandrite and Malachite tend to be larger and more monstrous.
- It is discovered by Steven and Garnet in "Keeping it Together" that the Homeworld Gems are using unknown methods to physically fuse the remains of destroyed Crystal Gems, a process which Garnet refutes as fusion due to there being no consent between the involved Gems. The process appears to involve leaving the Gems in extensive pressure and waiting for them to incubate and form a fusion.
- According to the Guide to the Crystal Gems, fusion of two different Gems was seen as rare until Ruby and Sapphire formed Garnet.
- So far, only four fusions have shown their weapon: Garnet, Opal, Sugilite, and Sardonyx.
- In "Cry for Help", Pearl was able to keep her activities a secret from Garnet when they were fused for the second time, implying that fusion does not mean that the link between the Gems that make up the Fusion Gem share everything while under one mind.
- According to the Guide to the Crystal Gems, if Gems of the same gemstone fuse, they will simply create the same Gem. Only Gems of different gemstones can fuse into a different Gem.
- According to Rebecca Sugar, the reason why the Gems fuse can determine height of their fusion. Since Amethyst and Pearl fused to save Steven, their fusion was small, perhaps slightly taller than Garnet, while Malachite's reason for fusion was to destroy the Crystal Gems, which would make her slightly shorter than Sugilite.
- It is implied in "The Answer" that Ruby and Sapphire are the first known Gems of different gemstones to fuse.
- Fusions can shape-shift, as demonstrated by Garnet in "Together Breakfast" and "Keep Beach City Weird".