It’s one of those nights where I google into the deep hours of the night and try to determine the composition of the Arkenstone! The film gave us a sort of fantasy glowing opal, but my results were different. In summary:
~=Cerussite is a ridiculously sparkly, heavy, and poisonous mineral associated with veins of gold which can also be poisoned. And it (with creativity) could conceivably be found in an extinct volcano.=~
Of course, there’s rather more where that came from because I don’t know when to stop when doing fantasy geology.
Pictures, Tolkien quotes, and geology under the cut. Warning: