New evidence for the Arkenstone-Silmaril case

[Tirinor]

As far as I know, volcanoes are not good for mining. Although useful for forging rings of power, a volcano does not yield any useful of precious rock but only cooled lava.
If this is true, it would be a poor place for dwarves to make their home. And since they did make their home there, it could be assumed that it is not an extinct volcano. Of course this all depends on how reliable my geological knowledge is. This doesn't nessessarily destroy the theory. Dwarves, as we all know, delve deep, and regardless of if the lonely mountain was a volcano or not, dwarves are capable of mining to a place where a silmaril could have ended up.

Also, I may be gramatically in error, but could "found or brought together" be logically divided into "found together or brought together"? if so, the prophesy does not pose an obstacle to the Arkenstone being a silmaril, since the arkenstone/silmaril was not together with any other silmaril.