What is it? Set 380
 

Lodestone.

That's it! At least that's the answer for what the heavy part is, though
it doesn't really address what it was for.

The ring on the top looked like a winding key - is it just for attaching
it
to your key chain?


The photo was taken in a mariner's museum, I don't know how old the item is,
my guess is that the ring was used to hang it from a string.