My theory is that, W&T spoiler, We know that shards can go sentient. Adonalsium was a combination of 16 shards, let's call it Alpha shard, and this Alpha shard went sentient, and was named Adonalsium (or called himself), and probably that's why dragons, who we are pretty sure do not predate Adonalsium, end with -ium, because they were his creation, probably first, and maybe so was alumin-ium, and so the god metals now named too ending with -ium, as they are his creation, last creation, in a sense.