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.