The Pinhoe Egg, of which the book of the same name is eponymous, is a mysterious object that is found in the cluttered attics at Woods House, the historic home of the Pinhoe clan in the village of Ulverscote, near Chrestomanci Castle.
The object is sort of eggshaped, but ambiguous when it's discovered by Cat -- Eric Chant -- the future Chrestomanci. He's immediately struck by how immensely wonderful and important it is, and when he asks Marianne Pinhoe about it, whom he's accompanied up into the attics in search of her cat Nutcase, her reaction seems to suggest that it's a strange old thing that nobody ever wanted. Cat asks if he could have it, and Marianne, who is the oldest child of the person who seems to be technically the owner of the house, tells him he may. So Cat takes the egg home to the Castle, absolutely mesmerized by how amazing it is. And his decision to take it and treat it with the reverence and care it seems to deserve touches off all the subsequent events of the novel.