Are certain relics rarer than others, or is it all random?

I do know that some relics don't appear in the shop, Wich is why they seem much rarer, but this isn't about that, there are certain relics or times i feel like some relics just never show up, like ether (or any of the Magnum opus parts). For me, I've seen ether like 4 times.

Or am I just feeling it cause I be looking for those specific relics Wich is why I notice when they aren't there?