Probably from any number of treasure hunting stories.

I like the idea to some extent but I wouldn't really care if it wasn't included in the game - I find all these "Mass scavenger hunt to find x amount of a miscellaneous set of objects for no particular reason" features to be pretty tedious, especially as there's always and achievement (a word I hate using when talking about RPGs) revolving around them. But hey, each to his own.
As for a reason/object I would suggest some ancient Nordic Artefacts or Jewels. There really isn't any need for a reason or even an introduction to them for the player, just have it as a nice 'hidden' feature. Perhaps there could be a book or two on the subject ingame, in taverns nearby you could overhear NPCs talking about them, or a 'Rumor' topic from the innkeeper could be something like "Had a lot more of the treasure hunting type through here recently. Folks reckon there's some kind of ancient treasure out in those woods." (No, I don't know why the innkeeper sounds like a cliche cowboy from a bad western film xD )

Squalled.

I like the idea about finding info in books. Maybe the auther have writen clues on where to find one of them and have heard of a reasercher that's interested in them.

Indeed, would make searching out books pretty fun.