as a kid growing up in the midwest, there were a lot of Black-Walnut trees around. we found out that if you picked up the walnuts off the ground and threw them against a wall, they left a dark stain that was nearly permanent.
why does it leave a stain? iodine. its very high in Iodine content. how high? i have no idea. i tried to find out via multiple internet searches but was unable to. i cant find one study that fixed the amount of Iodine in Black-Walnut s. of course it tastes so awful it would be hard to do enough Black-Walnut everyday to get your iodine.
however, its also possible that this high Iodine content plays a part in black walnuts legendary anti parasitic properties.
this was pointed out to me by plzchuckle, so thanks to her.