No so long ago, arms dealer Mike D'Ambra had a smallish, very ornate, flintlock pistol which was a diamond. It was made of brass, (or bronze), and the only discernible ferrous metal on it was the facing on the frizzen. Not sure whether the internals were brass, or not, but this was one fine 1700s gun probably made in Italy, or France. So, there is at least one such rarity in the world.
Dick