Brother Fir seeks the Heartstone for Crispin's coronation. However, Fir knew the Heartstone's weight and feel. When he let Hope play with it and it didn't drop out of Hope's paw, Brother Fir declared that the Heartstone had gone missing, and been replaced with a fake, which was presumably made by Aspen and switched by Husk.
Hope accidently finds it at the end of the book when he is giving the people on the beach shells. It is revealed that the Heartstone was thrown by Husk into basalt that was loaded onto a ship and carried away from Mistmantle, it then returns on the King's Arrow, the ship Urchin comes home to Mistmantle on.