The Shifting Sands by Emily Rodda

When the seven gems of the magic Belt of Deltora were stolen, the evil Shadow Lord invaded the kingdom and enslaved its people. Determined to rid their land of the tyrant, Lief, Barda and Jasmine are on a perilous quest to find the lost gems, which are hidden in fearsome places throughout the kingdom. They have found three gems. Now they must seek the fourth, hidden in a shimmering, barren waste, kept jealously by an unknown guardian. Separation, confusion and strange, terrible enemies await them in the ordeal of the Shifting Sands.