Some friends brought it back to us from a trip to the south seas. We're told it is the kind of thing used by cannibals to crack open people's skulls when they want to eat their brains.

Not sure why it made our friends think of us. (OK, I did have one remote family member eaten by cannibals in that part of the world, but I don't think they knew that...)
It is interesting and we don't have anything else like it.