Diana L Paxson 
Sea Star 
Book Five of the Chronicles of Westria

Julian – long-lost son of Queen Faris and King Jehan – has returned. Yet he still has to prove his heritage before the healing of the kingdom of Westria can begin. To gain the throne he must search out and master the four great Jewels of Power, lost in the magical cataclysm that took his parents. But Caolin, treacherous sorcerer and author of the great destruction, still lives. Only the Jewels stand between him and total power. If Julian dies and Caolin succeeds in his terrible ambition, Westria is doomed.