Oh, I certainly don't think Adonalsium is a metal. My statement is more in saying that Adonalsium is neither the source nor the sum total of power in the Cosmere. But is instead only a manifestation of a part of a much greater, older one - the God Beyond. So the '-ium' suffix isn't used to state that it is actually a physical metal, but instead is used to show the power that is Adonalsium is being held by a being known as Adonals.
I can further extrapolate (Theoryweaving, ftw) that should all 16 pieces be brought together correctly, you wouldn't actually have Adonalsium - you would have all the power and potential, but it would be open, ready for a bearer. So.... Hoidium, anyone? Naw, couldn't see things going that far, but I am sticking with the concept. As the Shards are pieces of Adonalsium, so too is Adonalsium a piece/large part of the power known as the God Beyond.
I guess what I am kinda saying is it looks like Adonalsium is a Macguffin. An extremely powerful, nigh omnipotent Macguffin, but still not the 'end of the road' entity it is being made out to be. I believe possessing and Bearing Adonalsium is very comparable to a Shard. And even with all that impossible amount of power, there is another deific level above.
Is there any WOB that specifically states that there was no Holder of Adonalsium? I thought that everything about this mysterious, broken thing has been kept in mystery, but if there is clear Words from On High about it, I will accept my wrongness. Hell, it is a theory, odds are completely for it being wrong.