The Opal Quest — Gill Vickery

When the DragonQueen's Jewels of Power were stolen by the six sister-witches, the dragons were banished from the land of Tulay. They took with them a human child as hostage. Tia has been raised by the dragons and considers them her family. But when she learns that her mother is one of the sister witches, she must prove herself to be a true DragonChild by retrieving the jewels, with the aid of her DragonBrother. In this second exciting adventure, Tia goes after the jewel that gives its possessor the power to shift shape.