Can you get any closer to it with the camera? Sometimes you can date it by looking at the way the links were made, or the way they were attatched to the medallion pieces. Right now I would be leaning towards a chain for clasping a cloak, only because it was my first impression.
Typically cloak clasp are short to keep the cloak up on your shoulders. But in some areas, it was fashion to let it hang behind you, and at the time of wearing armor and such 6 inch clasp would not be suprising for the cloak to fit over the armor, but most armor I have seen that intended a cloak for decrotive purposes had clasp actually on the armor.
This one could be for a riding cloak, or as said before could be a multi strand braclet, or a belt, dating it would make it a bit easier.
I doubt a necklace due tot he 6 inch span you mentioned, that would mean there was more to it, and they were usually longer.
A braclet maybe?