I assigned gems to Sonic and Shadow because... I can I guess?

Sonic is Aventurine--aventurine has the metaphysical property of promoting self worth and self confidence, which I think is a perfect fit for Sonic. It also sparkles, I have a crystal at home~

Shadow is Prasiolite. I picked it because it's a green quartz, idfk.

In Steven Universe all the quartz characters seem to have this spindash ability, so I figured these guys should be quartz as well.

When they fuse they become Citrine, which is a gold-yellow variety of quartz. Picked because I wanted their fusion to resemble their Super forms.