They are artifacts manufactured by land creatures whose ancestors last lived in the oceans 350 million years ago.

The practical examination took place in the afternoon on the lawn on the edge of the Forbidden Forest, where students were required to correctly identify the Knarl hidden among a dozen hedgehogs (the trick was to offer them all milk in turn: Knarls, highly suspicious creatures whose quills had many magical properties, generally went berserk at what they saw as an attempt to poison them);