my sister has a theory that the more compatible and cohesive a gem fusion the more humanoid it looks.
Gems like Garnet and Rainbow Quartz are fairly normal looking because both Ruby & Sapphire and Rose & Pearl are so close. Their relationship is stable so their form is.
Opal, Sugilite, Sardonyx, and Alexandrite on the other hand have extra parts. Opal only has two extra arms, which makes sense. Amethyst and Pearl are opposites but they do deeply care about each other as shown in “On The Run”. The other three have three or more gems in them and a respective number of extra eyes and limbs. This is understandable because as anyone who has ever worked in a group before knows, even close friends have a hard time agreeing with each other all the time.
Which brings me to…
Malachite was formed by anger between two gems who hate each other. Their form is dysfunctional because their relationship is dysfunctional.