Any fantasy setting that doesn't have magic crystals is doing something wrong. It's like having a cosmic horror setting and not doing anything at all with the Moon: It can definitely be done, by why would you?

Also, did Art's expression just soften? I don't think I've ever seen her looking less grumpy than she does in that last panel.

The rodent-wielding girl seems to be too active to need meth... I mean magic crystals.

I like the thinking of the Purple Beast - they are definitely most useful as a commodity. I wouldn't consume them, not worth it. But if you find some sucker willing to pay for them - why not.

Are those black crystals?

These kind of look like black crystals, something feels very wrong about them, especially the whole "Spirit flows into them" part, making me think they are... hungry.

Well, they're sure to become more valuable once you actually do find some utility for them. "Neat gizmo, no use for it tho" is not the best sales pitch.

I mean, that can explain why there are magical artifacts like Archibald's amulet, and Piper's staff, been said before they are just humans.

The gems in those artifacts would have been charged by the Mage King. Dunno about the capacity of those or how would they recharge, Mage Kingdom bureaucracy, whatever.