Rock cake

A kind of fruitcake. It's supposed to look something like a rock, and even to have a hard surface, but not to resemble a rock quite as closely as Hagrid's version seems to. Hagrid's rock cakes are exactly like eating rocks, as they are so hard that they could break teeth intstead are being a lovely treat. Of course, Ron, Hermione and Harry don't tell Hagrid this as it would hurt Hagrid's feelings.