Some more thoughts on the identity of smokestone:
1) I agree that it is unlikely to be obisidian. As Shardlet pointed out its not a crystal, and it would be strange for nine of the polestones to be crystals and the last to be a glass. Also Dalinar uses obsidian in a description in the Purelake vision, if obsidian=smokestone he would have said the latter. As for the volcanic activity required to make obisidian... Plate tectonics aren't a factor on the super continent but there are probably a few geologic hot spots where volcanoes would form.
2) I also don't think smokestone would be unique to Roshar/formed through a process we don't have on Earth. This kind of goes back to my point above, it would be really strange for nine of the polestones to be formed one way and the last a different way.
3) I also think, like Windy suggests, that we can discount it being "magical" in nature (like atium). In TFE they remark that "atium is different and that it doesn't really fit" whereas smokestone does.
I personally think it is just smokey quartz, like the link I supplied in my previous posts suggests.
On the soulcasting of "new" things:
Soulcasting, especially soulcasting of non-Essences, requires an intimate understanding of the desired result. It wasn't enough to know "strawberries exist" when Jasnah was trying to soulcast the jam. She had no clue what it was supposed to be like and so it turned out horribly. Though here's a thought, perhaps in a way it is like Forgery? The more "plausible" the soulcasting result is the more likely it is for it to work?