For theorizing purposes...

[Qarlin]

Atium's rare enough as it is. Somehow I have to doubt someone made a dagger out of it. Besides, someone would have to die for him to not die when the spike was stabbed into him. That sort of thing doesn't happen casually. Hemalurgy is done on purpose, not accident.

Also, Atium comes only in beads. Beads in special geodes.

And finally, he didn't work in the Pits. The scars on his arm that made Vin think that he had worked in the Pits were from him cutting himself to lessen the voice in his head.

I'm not ruling out the idea of the spike being Atium, I just don't think it's likely; that would take a lot of Atium.

[darxbane]

Atium, like any other metal, can be smelted and formed into any shape you want.  Examples: The Lord Ruler's Bracelets that Vin tore from him were made of Atium (Elend sells them in book 2).  It would take a lot of atium, but TLR did comment on how difficult they were to replace.  Also, there is the possibility that only the outer layer of the spike be Atium, and the rest be some other metal.  When Vin burns the Atium covered lead bead, she does get a small effect from the coating at first.  In the case of Hemalurgy, you may be able to get away with it.  It is possible, and he is so comfortable burning it.  The only other thing it could be is Brass.  I don't know where I read it, but EOUL mentioned something about Zane Soothing her almost constantly, it is just never mentioned.

[Wielder]

Atium's rare enough as it is. Somehow I have to doubt someone made a dagger out of it. Besides, someone would have to die for him to not die when the spike was stabbed into him. That sort of thing doesn't happen casually. Hemalurgy is done on purpose, not accident.

Also, Atium comes only in beads. Beads in special geodes.

And finally, he didn't work in the Pits. The scars on his arm that made Vin think that he had worked in the Pits were from him cutting himself to lessen the voice in his head.

I'm not ruling out the idea of the spike being Atium, I just don't think it's likely; that would take a lot of Atium.

Oh, I was just throwing out random ideas haha.  I completely understand that a atium spike/dagger would take quite a lot of atium, but like others have mentioned, he seems to be able to burn it for quite a long while.  I understand that he has a stockpile, but atium burns out quickly, so even a stock pile would run out rather quickly. 

Also with the Pits, good call.  I could swear that his father had mentioned it at some point, though.  It's been a while since I read it, so I there is a high chance I completely missed the target. 

I don't know where I read it, but EOUL mentioned something about Zane Soothing her almost constantly, it is just never mentioned.

You read that in one of the latest annotations.  However, I don't think hes got anything magically special in regards to steel pushing or soothing--Zane is just a very subtle character and his powers follow suit.