The Starflash Opal — Linda Talbot

Swot and Gemma - two unsuspecting schoolchildren - are chosen by enchantress Zophenas, to help her escape from the Underearth. They have to bring her three items that might do the trick; the soul of Waldo the warlock, the magic ointment that enables a witch to fly and a woman who has turned into a two-tailed cat. In return they will receive a starflash opal and six wishes.