The Pearl of the Soul of the World: The Darkangel Trilogy, Volume III by Meredith Ann Pierce

All the world’s wisdom and magic resides within the iridescent depths of a small white pearl. “All my sorcery,” the Ancient Ravenna had said of the pearl. “It is left to you to save the world.” But is the pearl powerful enough to enable Aeriel to defeat the White Witch? Aeriel’s people have assembled an army--led by the redeemed darkangel Irrylath--and are soon to attack the Witch and her darkangel sons. But their cause is hopeless unless Aeriel can unlock the mysteries of the pearl . . . and of her own destiny.