Heart Stone by Lisa Lowell

After Tanzaa is slated to become one of the King's many wives, she soon discovers the reason: the King covets her untapped gifts of magic. But she is in love with someone else: Dayvian, a simple musician who has her heart.

After Rachel - one of the Wise Ones - tells them that Dayvian is to be one of their magical order, their life takes a drastic turn. Pursued by forces both natural and unnatural, they escape the court and seek the Heart Stone: an artifact that will fulfill Dayvian's destiny.

But with all odds against them and time running out, can they prevail?