sardonyx is exactly the same as sugilite.
both are immensely powerful, enjoy showing off, have extreme opinions of themselves, and are ultimately not particularly great people.
the difference in how they act, and how they're portrayed, is due to the major differences between pearl and amethyst. this episode very heavily confirmed that fusing with garnet is a massive power high for the both of them, and they both very much are subject to enjoy it.
however, amethyst tends to be a much more controlled person. she's... scrappy, and she can sometimes lose that control, but she does her best to "only feel how i want to feel". she also, naturally, buries a lot of her emotions very deep, and she is more heavily affected by what others think of her than she wants to be. pearl is massively obsessive, and massively depressed. she needs to be dependent on another person to take care of or to look up to, in order to help her self-esteem. she very much needs it, and she's liable and willing to lie to herself and others when she feels like it's gone.
ultimately, this explains why both of their fusions with garnet went so wrong.
amethyst did not obsess over being sugilite. she clearly loves being the fusion, and she's very excited to do it again, but she doesn't feel like she needs it as a person. however, once she has it, she has a very hard time letting go. she loves the sheer power, and along with garnet thinking of herself as an individual and not two people, this means sugilite very quickly stops any and all desire to split apart, and becomes extremely offended when the idea is even brought up.
meanwhile, pearl isn't prone to try and overpower garnet, or anyone, like that. but the fusion not only makes her much more physically powerful, and she does seem to be the "weakest" gem in that regard, it also makes her feel very good about herself by proxy. sardonyx is pearl with her insecurities dropped and her inner ego at full display. she couldn't keep it going as long as she would have wanted, possibly because sardonyx feels like being showy for a short time is better than being around all the time, but she was just as trapped by the high as amethyst was. so she had to invent a way for her and garnet to fuse constantly.
sugilite was physically destructive and prone to give into amethyst's inner anger, sardonyx is an egomaniac who shows very little opinion of other people.
and they both caused very big problems.
after this, garnet could very easily be strained from the entire team, not just pearl. amethyst not only tried to defend pearl, but also did indeed confirm that the fusion is basically a pleasure trip. garnet, who has such a high opinion of fusing, is probably feeling very personally betrayed by the both of them.
this was a big deal. this is why this is the week of sardonyx. it's not a week of her appearing, it's about how she has severely disrupted the crystal gems, and we have no idea when or even if they'll ever have the same bonds of trust as before.
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 ^it's a tumblr link oh geez^ oh man is this a steam profile
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