Diana L Paxson 
Jewel of Fire 
Book Seven of The Chronicles of Westria

There were four Jewels of Power — one for each of the sovereign elements: Earth, Water, Wind and Fire. Whosoever controlled the Jewels controlled the destiny of Westria. Long ago they were lost in the chaos of the war of magic and the land drifted, leaderless into decline. Prince Julian, rightful heir to the throne of Westria, has recovered three of the four. Only the last, the Jewel of Fire, buried in the burning heart of a volcano, eludes him. Yet before he can restore the Kingdom to peace, Julian must recover the Jewel and overcome his terrible enemy the Blood Lord Caolin.