Sheilajeanne, you must have a very different type of walnut than my tree as mine are pretty easy to crack, unlike my macadamia nuts, the only way I can crack them is in a vice! In fact there is only one bird that can crack them and that is the Black Cockatoo (must have one hell of a strong beak!!!). The White Sulphur-crested Cockatoos, on the other hand, do a great job of stripping my walnut tree. And yes, the ripened husks stain hands wonderfully!
As DaleksInc said I suspect most leather goods way back then were just left to age darken naturally.