What if Atium wasn't matter? It has mass, and has metallic properties, but isn't matter?
Here's what I'm thinking: The three realms (Physical, Spiritual, and Cognitive) each have things which are primarily "of" that realm. Reflections of these things exist in the other Realms, and can interact with the other things in that realm, but aren't really there. So matter is anything "of" the Physical Realm, while the god metals, shardpools, and mist are of the Spiritual Realm.
This would mean that Atium has physical presence in the Physical Realm, but it isn't actually composed of the fundamental particles that define matter.
It's kind of a Plato's Caveish kind of idea.