Also, I have a question. Since there is a balance, and there is never more Ruin than Preservation in total, does it mean that if atium comes back to earth after it's used, would Larasium do so as well? It takes hundred years for atium, and if larasium would, I think we have a storyline for the next possible book in Mistborn world, where there are only Mistings, and suddenly, the possibility of becoming a Mistborn reappears. How much chaos would that make and how people would react to it, it's a perfect background for a story of Mr. Sanderson.
This would explain why Vin was so confused when she thought about that nugget of Larasium just laying there. It could be that, like Atium, it slowly returns to the world. After all, it doesn't really make sense that the Lord Ruler would just leave one Larasium nugget laying there "just in case". He did have incredible foresight, but this just seems silly.
Another explanation for the nugget laying by the Well is just that Preservation, after stabbing Elend and realizing that Vin would release Ruin anyway, took part of it's "body" and placed it there to save Elend.
I'd rather the first be true, just because it sounds more exciting that more "pure" Mistborns, with exceptional strength like Elend's, could be born.