Diadem is a series of fantasy novels by John Peel.
The series takes place on several different worlds, collectively referred to as the Diadem. Travel between worlds is accomplished through the use of magic portals. The physical distance between worlds is irrelevant. Portal travel is based on the mystical geography of the Diadem, which is divided into five layers. The layers are a little like the layers of an onion and a drawing of them is the logo of the Diadem series and is on every book. At the center is a single world called Jewel, followed by the Inner Circuit, Middle Circuit, Outer Circuit, and Outer Rim. Magic is strongest on Jewel, and progressively weaker the "farther" a world is from the center. In the Outer Rim, magic barely functions at all.
The main characters are three adolescents from worlds on the Outer Rim. Each has extraordinary talent for magic. Due to the weakness of the magic on their home worlds, they do not learn about their abilities (or their mysterious destiny) until the events of the first book.
The son of Bad Tony Caruso, Score spent most of his life in fear of his father and the Bowery gangs. His mother died when he was young, supposedly a victim of his father. She was a strega,a wise woman, and may have originally come from Ordin, Helaine's world. At the end of the tenth book, it is revealed that Shanara is Score's mother. When his father was arrested and jailed, he was put into state custody, but escaped. Score was brought to Treen by S'Hee, an Orca Bestial. Score, after travelling to Dondar, bonded with his unicorn friend, Thunder, who is both the herd leader and father of Flame and husband to Nova. His main magic ability is to change objects from one thing into another. But he also has the abilities of sight, water, and the control over an object's size.
This easy-to-be-around joker is the best at magical fighting and the second best planner (Pixel is the best). He constantly fights with Helaine but later in the series he shows romantic interest in her despite his resistance to keep his feelings in check.
Score controls an emerald, amethyst, jasper, and chrysolite.
Helaine is a female warrior from a medieval world. On her planet, she was a noble, but still considered "just a girl" and therefore wasn't allowed to live her life as she planned. So she disguised herself as a boy with the alias "Renald." In this guise, she was able to wander the castle and learned to sword-fight like the other castle boys, eventually becoming the best among them. As we first see her, her father has planned to marry her off to Dathan Peveral, the imbecile son of a needed ally. After her guise as Renald was ruined by Oracle, she planned to run away. As she was leaving, the Shadows inhabited a group of soldiers sent by the border lords to kidnap her and hold her for ransom but she was rescued and brought to Treen by Rahn, a leopard Bestial. After reaching Dondar, she bonded with Flame, her unicorn friend, and the two are very close. Flame is the daughter of Thunder and Nova, and she seems to be the only person Helaine will open up to. Helaine's main magic ability is the power to foresee trouble. Her powers with the gemstones are levitation, communication, earth, and shape-shifting.