The Secret of The Crystal Fairies (Thea Stilton #7)

The Thea Sisters are just about to head off on their separate family vacations when they receive a call from Will Mystery. Once again, there is trouble in one of the imaginary kingdoms. A crack has been discovered in one of the maps in the Hall of Roses. The girls head to the Crystal Kingdom to help uncover the mystery. There, they find that a dragon has been wreaking havoc all over the kingdom. He has been turning all of their precious gems into stone and leaving a path of destruction where ever he goes. The sisters figure out why and help break the spell.