The Gemstone Chronicles Book Three — William L. Stuart

Aidan, Maggie, Nana, Beebop, and their elven friends have recovered the first two of four gemstones stolen from the Elven Bow. With the second stone - the Amethyst - protected by the Elven Scouts Alatariel and Lotheth and on its way to Keeper for safekeeping, the rest of group heads deeper into Celahir to rescue Aidan and Maggie's father, Mike, from the clutches of the witch Maeva and to recover the third gemstone - the Emerald. After a fateful encounter at Maeva's house, the witch turns everyone, except Aidan and Maggie, into stone statues to decorate her lawn. With the help of Dylan - a ghost boy they met in the village of Fearn - the children have to muster all of the courage and ingenuity they possess to find a way to retrieve the witch's heart from its hiding place and trade it for their family and friend's freedom. If they can do that, then they might be able to find the third gemstone by defeating the cockatrice...