So, i've been listening to some tracks these days and I noticed something:
Pearl and lapis' themes are basead on piano (I'm not saying that they are related, that's another theory I did) . Both share somo physical traits (Ex:Tall, skinny, pointy nose, hair)
Jasper and amethyst's themes (Amethyst doesn't have an especific theme, but drums are usually used in tracks related to amethyst) both have drums as a base, and these ones are obviously simillar (Probably because they are kinds of quartz)
I noticed that in the track "The mother" (Centipeetle's theme) we also got lot's of drum-based parts. Mabye uncorrupted centipeetle was simillar to jasper, or mabye a kind of quartz