Fusions are the product of two or more Gems who are emotionally harmonious with eachother, either deliberately through a synchronized Fusion Dance, or it could be a spontaneous event between individuals.
Fusion Gems, to their core, are made up the same way as other Gems. They are quite various in details depending on the Gems involved within their genetic code. If the Gems are of the same type, then it'll only result in the exact same Gem but a larger size, but if differentiating Gem types fuse together, the resulting fusion will either hold physical attributes of the Gems fused into it, or gain traits unique to its own physical body entirely. Gem placement, and colors are also effected by what Gems are involved in a fusion, as the Gemstones not only change color but are more numerous with more Gems fused together.
The size of the fusion depends on the Gems involved, and how many: the more fused together, the larger the end result. The height gained can be accelerated or slowed down depending on the size of the Gems prior, such as Garnet: who's made of two small Gems Ruby and Sapphire, compared to Malachite: who's made up of Jasper (a large quartz soldier), and Lapis Lazuli (A medium-sized water gem). The exact size limit for a fusion is unclear, though a fusion can exceed planetary sizes according to the information given for The Cluster, which is made of millions of gem shards. Fusions of the same type will not alter the end body from their own body originally, but when different types fuse, they often results in multiple arms, legs, or eyes (with a few exception as in Rubidot and Stevonnie).
The fusion is combined with both physical and mental states for each Gem involved. This means that the exact personality of a Fusion can be altered on the two Gems involved. However, Fusion interaction with other Gems outside the fusion is actually far easier, since the Fusion maintains a set of memories from both Gems involved. This kind of memory can end up affecting both Gems involved in the fusions, and even sharing memories one Gem might not know of, evident with Jasper seeing Garnet's memories while fused as Carnelian.
Practically any and all fusions do suffer from this sort of condition, especially fusions with conflicting personality traits that clash with eachother. Typically fusions would have conversations with themselves, and in dire circumstances, even fight for control within the fusion body, which could result in de-fusing.
Stable fusions are basically as they sound: a case where the Gems involved in fusions can remain together for extended periods of time with little to no trouble. This type is typically seen throughout the series, as fusions of this nature are focusing on a task, or have a basic mindset well enough to keep together. Typically they're used for just that and last for a few minutes to a few hours, to which the Gems will split up after the task has completed.
Unstable Fusions are a bit more precarious than Stable fusions are, and that the personalities conflicting within them could make extended periods of fusion harder to accomplish, almost becoming a struggle just to keep together, as with the case of Malachite. While more varied, they're the more likely type to begin with, especially in evidence for first-time fusers, and fusions like these don't really last for too long.
Instability in Fusions also increase with more Gems fused, as it becomes harder to keep together the more varying Gems are involved.
- Main Article: Cluster Gems
Forced Fusions mainly come from two or more Gems who're fused against their will, and result in a fusion that's extremely unstable and unpredictable. These fusions as well aren't able to be unfused, and as such become emotionally detached, lack of sanity, and behave more like Corruption Gems than regular fusions would. The word was first coined by Jasper after their fight with a Cluster Gem, thinking that Carnelian was a forced fusion as to how it occurred. The most prominent examples of this is The Cluster.
Perma-Fusions are Fusions of Gems who have little to no intent on splitting apart, and can remain together for days or even years on end. These fusions are the most stable type of Gem fusion, and are typically made up of Gems with an extreme bond strong enough to endure thousands of years of being together.
Mega-Fusions are Fusions that're made up of a high amount of Gems that can remain stable together. Gem fusions classified as Mega-Fusions typically have five or more gemstones fused within them, and despite so many Gems, can still operate as fair as other Fusions. Mega-Fusions though are very unstable, and are done mainly with members of a team with pre-existing connections, and even then most of the time it usually lasts as long as a typical stable fusion.
Fusions' abilities are as various as those of Gems themselves, as like their physical bodies can vary depending on the Gems involved. Typically though, the larger the fusion is the stronger it becomes. Gem weapons are also affected by this, and are fused together to create a new weapon itself, though the fusion is capable of using their counterparts' gem weapons and abilities to their disposal if they so wish to.
While unconfirmed if Gem DNA is shared between Gems, it is possible that fusion between Gems and other species (Stevonnie, Ammie, Earthstone, and Smoky Quartz) are able to inherit Gem DNA in fusion practice to the point of becoming half-Gem, as with Connie Maheswaran.