The Diamond Princess Saves the Day (Jewel Kingdom #4)

Four girls are about to become princesses! But can they keep their kingdom safe? In the Jewel Kingdom, four girls rule: Roxanne is the Ruby Princess, Sabrina is the Sapphire Princess, Emily is the Emerald Princess, and Demetra is the Diamond Princess. Some of Princess Demetra's subject have become trapped inside a mountain. She must risk everything--including her crown--to rescue them!