Underwater volcanoes are dolomite, I think arcane scoria has something to do with the dark sea? Really says a lot about just how useful the new potion types are when pretty much nobody knows where to find their respective catalysts.
Edit: checked the wiki, volcanic dark sea islands.
Rare drop from breaking rocks I think, and stupidly rare at that.
I do know how to get volcanic crystals, though. You need to break gravel piles (can be orange, light or dark gray), like the ones found on Ravenna or after port Mistral burns down.