spectacularfail

From everything I've seen, the metals are indeed physically consumed/burned away; I believe there's WoB saying if a nicroburst touched someone compounding (and thus allomantically burning) through an earring or bracer, the earring itself would burn away in a flash of energy. The recent post in the forums where a chemistry major discussed the chemistry of metals in mistborn said that, in essence, the godmetals were made of pure investiture mimicing atomic structure (i.e. investiture protons, neutrons, electrons). This is why atium was considered the "body" of ruin; it was his shard's investiture consolidated in the Physical realm, so when it was burned, it was dispersed into either a realm Ruin could not access, or it was dispersed in a form Ruin could not use. If I'm remembering correctly, this was WoB from a discussion the Chem major had with Brandon, but it could be wild speculation.

If I remember correctly, the godmetals (and maybe all metals, but I'm not sure) once burned transfer into a "raw energy" investiture state of sorts which eventually pools back out in the form of metal deposits in the ground (Pits of Hathsin). So, presumably, there is more atium in the ground somewhere, but I do remember a quote from the first trilogy about the time it would take for the Pits of Hathsin to recover being very high, so maybe not. It's possible that Harmony geographically moved the shardpool to a point that it is not feasible for anyone to access the atium.