More old-timey chemistry from Appendix N author A. Merritt:
What of that radiant unknown element upon the moon mount Tycho? What of that element unknown to us as part of earth which is seen only in the corona of the sun at eclipse that we call coronium?
-The Moon Pool, 1918
The Moon Pool is as much sci-fi as it is fantasy or horror, despite its author's influence on H. P. Lovecraft. It draws on the archaeology and chemistry of its day, much of which is delightfully wrong. Failed historical science is a great source for fantasy.
So apparently, in the 19th century, scientists thought they'd discovered a new chemical element which only existed in the the sun's corona. They called it "coronium." It turns out they were misreading their spectrographs and they were just seeing highly ionized iron. It's a great name, though: as selenium's name suggests "moon stuff", coronium is "crown stuff" or "sun stuff."
We all know that, as mithril is super-silver that trumps steel, there must be a super-gold that trumps mithril. Maybe it's coronium. When the gods take up arms, I bet they draw shining golden swords: at least, in Greek myth, Haephestus is always making gold weapons and armor for people (a shield for Achilles, a breastplate for Hercules, bow and arrow for Apollo.)
How rare is coronium? Dwarves mine for mithril, but can they even find a coronium vein? My guess is that such weapons are only the gifts of the gods.
And what of that other unknown element we find glowing green in the far-flung nebulae—green as that we had just passed through—and that we call nebulium?
-The Moon Pool
Merritt mentions another fun fake element: nebulum (or nebulium or nephelium), another spectrographic mistake, "discovered" in 1864 by William Huggins. Huggins thought it was an element that only appeared in nebulae. It turned out to be ionized oxygen. Again, nebulum is a great name for a magical material: "cloudstuff."
One more Lovecraftian detail about nebulium: its spectrographic light signature wasn't identified as oxygen right away because scientists thought it was impossible that such super-ionized atoms could exist long enough to emit light. Such an unearthly electron state, impossible except in the voids between the stars, is seriously called a "forbidden line". It's forbidden light! That's reminiscent of Lovecraft's story "The Colour Out of Space", where unusual cosmic light causes all sorts of eldritch trouble.
Nebulium seems like a great counterpart to coronium. It might be used to forge the weapons of the evil cloud giants, or it might cast invisibility on its owners, or it might radiate darkness or even madness. It could even be the strange black metal of drow weapons. It might be the harbringer of beholders, grell, and other creatures of the far realms.