>>102104404>Is there any rhyme or reason to fusion in Steven Universe?
Only as regards the qualities of the characters. Pearl and Amethyst have a strong sisterly bond, and in Opal their differences complement each other, making the fusion easy and herself very relaxed. So relaxed, in fact, she tends to forget important things.
Amethyst, on the other hand, plays into Garnet's desire to be free of the weight of responsibility that bears down on her, making Sugilite irresponsible and careless.
With Pearl it's the opposite: Garnet's will power allows her to be momentarily free and express her hypercompetence freely, making Sardonyx a cheerful showboat.
Interestingly, Amethyst and Steven are both very insecure about their own abilities, but very willing to help the other; consequently Smokey Quartz is highly competent, but still insecure about her(?)self.
Like if you combined a ruby and sapphire in real life, would it make a garnet?
No. Ruby and sapphire are in fact the same mineral, aluminum oxide or corundum. They are only different in color due to other mineral inclusions (chromium in ruby's case). Garnet is a silicate compound, so actually more closely related to the quartz group.>Or do they just pick gems at random for what fuses into what?
It's dramatic necessity.