The Search for Treasure (Geronimo Stilton The Kingdom of Fantasy #6):

I, Geronimo Stilton, was shocked to find myself back in the Kingdom of Fantasy for a sixth time. I was needed to fulfill the Ancient Gemstone Prophecy!

The evil empress of the witches was after the Royal Sapphire. If she united it with her Royal Ruby, it would give her immense power. The entire Kingdom of Fantasy was in danger! It was up to me to find the two gemstones first, with the help of new friends -- and this time, I had to travel in disguise. Cheese niblets! What an incredible journey!