Magic subject: a specific special metal or special metals in general
I'll do you one better, Special Materials™ in general: the more we get, the more silly they get, because it's really hard to strike a balance between how Precious and Special they try to be and how WotC flaunts it on cards to highlight its existence.
It's hard to sell me on how priceless Etherium is, or Halo, or Cosmium, or Powerstones, when they show up on every other card in the set and people are pretty liberal with their use. Hard to believe Darksteel is so rare when there's an entire foundry made of the stuff. Ixalan's story actually handled that a bit better than the card with Cosmium, I feel, since it wasn't treated as particularly special or precious there, even if it was sacred.
Generally, I think the Special Materials are better when they aren't a main focus, just a worldbuilding element in the background. Darksteel did that relatively well with a mechanical tie-in, Mizzium isn't flaunted left and right, Tyrite did it really well because it only appears a few times, and most of them are the same weapon. Aether is alright because it doesn't pretend it's particularly rare, but Halo isn't with how liberally it was used. Despite not appearing that much, Benalian enchanted Stained Glass was overused, I think, since it's supposed to be this precious family legacy material that only exists in small quantities, plastering it everywhere on all cards of the faction cheapens it (even if it looks fantastic.) Etherium has a canyon between how precious it's touted to be and how much we see of it everywhere on Esper.
If you're trying to sell a Special Material, don't plaster it everywhere if you want it to feel precious and special.
