I kinda randomly decided to whip up this fusion chart for the Gem characters from Steven Universe (along with my own Gem OC's Diamond, Emerald, Aquamarine, Topaz, and Onyx). It's not really finished--as you can see--but I laid down a few rules.
#1- we know for a fact that all Gems can fuse, but we'll assume Gems can't fuse with themselves...as far as we know yet.
#2- Fusion Gems, like Garnet, can still fuse with other gems, so--for the sake of argument--let's assume that fusion gems like Sugilite and Opal can also fuse, possibly with each other.
#3- this is only for TWO GEMS at a time; even though Alexandrite is on here, it's just to use her as a component. I also left out the fusion of all four of them for two reasons: 1- we don't have a definite picture of it, or a name, and 2- as previously stated, this only covers fusions of two.
#4- for the sake of simplicity, since Steven has Rose Quartz's gem, let's just assume he also has all the same fusion combinations as Rose.
#5- the only official gem fusions listed here (so far) are Opal, Sugilite, Alexandrite, Garnet, and Malachite (Jasper+Lapis), with the exception of OC fusions like Olivine (Diamond+Emerald), and this is entirely speculative. If and when canonical fusions (such as Garnet and Pearl) or Gems reveal themselves, the chart will be updated/altered to reflect this. Since Stevonnie (the odd Steven/Connie fusion) isn't technically a Gem, I just chalk it up to it being an anomaly only Steven, a half-Gem hybrid, can do so thus it's omitted from the list.
#6- since it's questionable whether COMPONENTS of a fusion can fuse with their own fusion, such as Ruby or Sapphire fusing with Garnet, we'll ignore these combinations; unless some form of Steven Universe canon (the show itself, a comic of some kind, official creator word, etc.) contradicts this, we can assume, for the moment, that a gem fusing with a fusion created from itself is impossible.
#7- there won't be any fusions of OC gem fusions like the aforementioned Olivine, since they don't exist in canon and thus they won't be added as components to the chart like Opal and Sugilite. Also, you can suggest your own OC fusions if they are speculated fusions of canonical characters, but not fusions with your own or someone else's OC's. Similarly, any fusions with my OC's Emerald, Diamond, Onyx, Aquamarine, and Topaz are off-limits since I'll be updating the chart with their theoretical fusions at a future point in time.
Feel free to leave any suggestions in the comments below.