Fusion Gems

"Fusion Gems" are the product of two or more Gems who fuse together to create a generally larger and more powerful Gem, entirely separate of the two or more Gems and often only done in case of an emergency. First demonstrated in "Giant Woman", as of this episode, all known Gems can fuse.

Appearance
Fusion Gems have the appearance of the two or more Gems they fused with. This means that the pigments, clothing, weapons, hair, and overall body shape of the Gems are combined into one general look. Also, each of the gemstones appear in the same locations of the Gems they fused with, such as Opal having gemstones on her head and chest; Sugilite having gemstones on her hands and chest; and Alexandrite having gemstones on her head, chest, and hands. The pairs of arms on a fusion Gem directly correspond to the amount of Gems that take part in a fusion. Prime examples are the fusion Opal from "Giant Woman", and Sugilite from "Coach Steven", who both have two pairs of arms, since two Gems took part in each fusion, and Alexandrite, who has six arms for three Gems fusing together. Many statues of fusion Gems with multiple pairs of arms are seen in, or depicted, on ancient temples that belonged to the Crystal Gems, showing that multiple pairs of arms are a common trait among fusions.

About
Fusion Gems are formed when two or more Gems perform a fusion dance to sync their bodies. When performed correctly, the gems will begin to glow and turn into an amorphous mass of gem energy before forming the full body of the new Gem. If done incorrectly, such as being unable to sync entirely, the two Gems will be ejected before finishing the entire transformation. It should be noted that when the Gems fuse, not only their bodies, but their minds as well, fuse into one, creating essentially an entirely different being as opposed to two minds sharing a single body, although the fusion of Steven and Connie and Jasper and Lapis Lazuli was shown to have separate thoughts with the latter pair showing it is possible for one mind to take over the body.

Culture
The Crystal Temple has an appearance of a typical fused gem. In Jail Break, Jasper derides fusion as a "cheap tactic to make weak gems stronger" while Garnet states that the fusion of Ruby and Sapphire is a symbol of love. Pearl implies fusion is rather intimate in Coach Steven and Alone Together; she tries to cover Steven's eyes while Garnet and Amethyst performed their fusion dance and says Stevonnie might be considered "inappropriate".

Abilities
When fully fused, the new Gem becomes a larger and significantly more powerful version of the two Gems that will incorporate the powers, strengths, appearances, weapons, and abilities of both Gems.

Weapons
When fused the new Gem can both dual wield their gem weapons as well as fuse these weapons into a single, more powerful weapon. These new weapons can use the abilities of all weapons that were fused such as Opal's bow which is made of Pearl's spear which is used as the body and arrow and Amethyst's whip as the string, and Sugilite's flail which is made of Garnet's clasped gauntlets as the ball and Amethyst's whip as the chain.

Standard Gem Fusions

 * Garnet (Been fused since before "Gem Glow", but revealed to be a fusion in "Jail Break")
 * The fusion of Ruby and Sapphire
 * Opal (Revealed in "Giant Woman")
 * Fusion of Pearl and Amethyst.
 * Malachite (Revealed in "Jail Break")
 * The fusion of Lapis and Jasper.

Multi Gem Fusions

 * Sugilite (Revealed in "Coach Steven")
 * Fusion of Garnet and Amethyst.
 * Alexandrite (Revealed in "Fusion Cuisine")
 * The fusion of Garnet, Pearl, and Amethyst.

Special Gem Fusions

 * Stevonnie (Revealed in "Alone Together")
 * The resulting fusion of Steven, a half-Gem half-human hybrid, and Connie, a full human.

Trivia

 * It's possible that Fusions can appear with different outfits.
 * The dance used to fuse is a mixture of each Gems' dance style, which are based on their personality.
 * If two fusions can fuse, that would possibly combine the features of both fusion Gems and their weapons, most likely resulting into a larger fusion, such as the one on the Crystal Temple.
 * Only six fusions are currently known: Garnet, Sugilite, Opal, Alexandrite, Stevonnie, and Malachite.
 * Stevonnie is the first Gem-Human fusion.
 * It is unknown what Pearl and Garnet's fusion will be.
 * The fusion Gem on the Crystal Temple has the most gems out of all of any Gem fusions, it has 5. One on the forehead (Pearl), chest (Amethyst), two on both hands (Garnet, some of the arms are broken off, so the second hand gem is missing), and finally the gem/house on the stomach (Rose). It also has the most arms, but some are missing, if they were still intact, it would be 8.
 * Alexandrite, a fusion between Garnet, Amethyst, and Pearl is very similar to the Gem that is on the Temple. However, Alexandrite seems to be missing Steven/Rose Quartz.
 * The "Fusion dance", displayed by the Crystal gems, is very similar to the Fusion Dance shown in "Dragon Ball Z", however more literal, in the fact that they actually dance, rather than complete a set of poses like the fusion dance in Dragon Ball Z, in order to become one.
 * This is similar to Mixels when they "mixit up".
 * So far, we know that Fusion Gems can be separated by suffering heavy damage (Sugilite) or when one of the Gems fused disagrees with the other (Opal).
 * Stevonnie seems to be an odd exception, since they separated while they were dancing in an attempt to embarrass Kevin.
 * As of now it is unknown whether or not Stevonnie or Alexandrite have a signature weapon.
 * Alexandrite is the only fusion with 6 arms and two mouths.
 * Alexandrite is the only Fusion Gem who has not shown their Fusion Dance.
 * Though the fusion dance was vaguely shown, Pearl and Amethyst attempted to fuse into Opal, but ultimately failed due to a lack of cooperation. However, it works a second time, but the dance sequence wasn't shown.
 * Gem Fusions can be made with humans, as shown in "Alone Together".
 * Although this could be an ability exclusive to Steven due to him being half-human and therefore "organic".
 * In the episode "Jail Break", it is shown that Garnet is a fusion.
 * From this knowledge, this confirms that fusions can fuse with non-fusion gems as Garnet has fused with Amethyst before to form Sugilite, and both Amethyst and Pearl to form Alexandrite.