Diana L Paxson 
Lady of Light 
Book One of the Chronicles of Westria

The four jewels of power had been created by a priestess in the days when the Covenant between the people and the Guardian of the land had been threatened. The heirs of her line held the Jewels – one of each of the sovereign elements – and through them, ruled Westria. In the sixth century of the Covenant, the Estates of Westria petitioned the King to marry, so that he might have an heir. So the King and his companions set out to find a Lady for Westria – a woman who would become both the mistress of his heart and the Mistress of the Jewels.