Walnuts are the round, single-seed stone fruits of the walnut tree. The brown, wrinkly walnut shell is enclosed in a husk.

Shells of walnuts available in commerce usually have two segments, During the ripening process, the husk becomes brittle and the shell hard.The seed kernels – commonly available as shelled walnuts – are enclosed in a brown seed coat which contains antioxidants. The antioxidants protect the oil-rich seed from atmospheric oxygen, preventing rancidity.