Been studying up on god metals, aluminum, silver, Adonalsium, and so on. I have many thoughts, and am going to try to make them all fit in this discussion topic.
First off, Brandon has repeatedly RAFO'd questions on relationships between aluminum and Adonalsium of basically any form.
He has said some interesting things about the difference between aluminum and silver. I find this WoB particularly interesting.
So some theories - not necessarily related, and occasionally in conflict...
Theory 1 - Is Aluminum Adonalsium's God Metal?
It's been theorized a great deal, at least in WoB. My gut reaction is no. But let's discuss reasons it could be.
It's found everywhere in the Cosmere, but is very rare, pre industrial revolution. If Adonalsium is or was a building block of creation and existence in the Cosmere, it would make sense that it would be found everywhere, but also that it would be rare. But it would also make sense why, post industrial revolution, it became very abundant. If you have the appropriate methodology, why couldn't you easily break things down to their most base element, the building block of creation, aluminum.
Additionally, aluminum is resistant to investiture. Could this be because, at it's core, aluminum IS investiture?
Reasons I don't like this theory:
It seems like aluminum has almost an opposite effect, rather than providing investiture, it resists investiture. This leads to an alternate theory that I MAY like better.
Theory 2 - Adonalsium and Aluminum - Opposites, Gods, and all the rest.
I think this is, in my opinion, one of the two most interesting ideas I'll bring up in this post, so forgive me if I wax long.
It has been indicated that there may be (or even explicitly is, depending on how you read the WoB) an opposite or opposing force to Adonalsium.
Let us look at the names. Adonalsium. Aluminum (or the British Aluminium). I'm not certain it's coincidence.
Maybe, pre shattering, there were two beings. Two opposing forces. Yin and Yang. Adonals (Ado), and Alumin. They were the co-creators of the Cosmere. Or perhaps, they were creators of two connected universes. A Cosmere and a Arccosmere (dumb math joke because I didn't know what the other name may be, can't find a good root meaning for "cos" that Brandon would conceivably have used). And what if Adonalsium and Aluminum are the god metals of these beings, or forces? Adonalsium is pure investiture, and Aluminium is that which resists it, or impedes it.
I really like this idea. Why don't we see the metal Adonalsium throughout the Cosmere? Because it all lives in the spiritual realm, fueling the investiture of the Cosmere. And Aluminium (ralkalest, brought by meteors) is present to resist it. Sent on meteors by this other being or force. Maybe the shattering was necessary and a choice, made by Adonals and the those who were there at the shattering, to help take investiture and magics and life across the Cosmere to prepare for a coming opposition.
Or alternatively, Where does silver come into play? It is not allomantically viable. It destroys investiture. So what if, rather than being at odds, Alumin and Adonals were truly yin and yang, working together to resist "the dark aether" the destroyer, the opposition, which is the progenitor of silver?
You can even look at Era 1 of Mistborn being Brandon trying his hand at this. Creating this yin yang, co-creator conflict between Ruin and Preservation as a hint for things that had happened, and that would happen, across the entirety of the Cosmere.
Theory 3 - Aluminum, Ralkalest, Ulidamium?
Could Aluminum be the God metal of Uli Da, the vessel of the Shard Ambition? Brandon has said that the splintering of Ambition has had disturbing affects on Threnody and far reaching effects in the Cosmere. What if that's how Aluminum can be found on any planet within the Cosmere? Because when Ambition was splintered, when Odium "tore off chunks of her power" those chunks were spread across the Cosmere. And maybe there is some relationship, some alloying, or some affect or relationship between Aluminum and silver, with silver being most well understood by the Threnodites because they were the first to face and resist the affects of the death of Ambition.
Theory 4 - Is Hoid a Cognitive Shadow?
Now let me preface this by saying I think we have evidence that the answer is no, based on previous WoB. I brought up a theory last May that maybe Hoid had been Adonalsium, or was an Avatar of Adonalsium, which received mixed opinions, and one argument made was that a previous WoB says that Hoid was born naturally. But Brandon, we've found, can say things that are "technically" true while not being completely forthwith, and that could be the case here.
This is the other of the most interesting (to me) ideas that I wanted to bring up in this post. Anyways, where does this idea come from? I read a WoB that I found very interesting...
Why is this such an enormous RAFO? Maybe because Hoid IS the cognitive shadow, that which remains, of the former vessel of Adonalsium, or perhaps part of Hoid is that cognitive shadow. And we know that not every cognitive shadow is the same. Kelsier's cognitive shadow remains largely the same, however Tanavast's cognitive shadow merged with the Stormfather, creating something new. Returned are the "same" being, driven by an intent, but without recollection or memory of how they came to be where they are. So, maybe the intent, the mantle of Adonalsium settled on an apprentice, friend, child, heir, of Adonalsium to continue to advance his intent and purpose. He can move throughout the universe because he is inherently connected to it. He is driven by an intent to save the Cosmere because he took part in its creation. He knows where he needs to be, but not what he needs to do, because Adonalsium saw into the spiritual realm and saw what would be, and is pushing Hoid in the right direction, while not having a complete understanding or knowledge of what to do once he gets there.