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 gems appear in the same locations of the gems they fused with, such as Opal having gems on her head and chest; Sugilite having gems on her hands and chest; and Alexandrite has gems 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 gem Opal from the episode "Giant Woman"  and Sugilite from the episode "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 fusion gems.

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.

Abilities
When fully fused, the new Gem will be a larger and significantly more powerful version of the two gems, that will incorporate the powers, strengths, appearences, 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.

Fusions
All known Fusion Gems:
 * Opal (Giant Woman)
 * Sugilite (Coach Steven)
 * Alexandrite (Fusion Cuisine)

Trivia

 * If two fusion gems can fuse, that would possibly combine the features of both fusion gems and their weapons most likely resulting into a larger fusion gem such as the one on the Crystal Temple.
 * Only three fusion gems are currently known: Sugilite, Opal, and Alexandrite.
 * So far both Opal and Sugilite have four arms.
 * It is unknown what Pearl and Garnet's fusion will be.
 * Sugilite is the only fusion gem with more than one pair of eyes.
 * 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.
 * 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.
 * So far, we know Fusion Gems can be separated by suffering heavy damage or when one of the gems fused disagrees with the other.
 * Alexandrite is the only fusion with 6 arms and two mouths.