G.E.M. by Jane Clarke
Royd is convinced that his friend, Gark, has everything. He's just got a space pod, and he's allowed to go to Mars whenever he likes. Not only that, he doesn't have an annoying little sister who always wants to play Hide-and-Bleep. Royd tries to keep his Green Eyed Monster under control, but at Gark's amazing birthday party, there's a booster bouncer, a magician from Mars, a dragon to light his birthday candles, and all the kids get to whirl around the rings of Saturn. Royd gets so jealous that his G.E.M. gobbles him right up! Can Gark help him realise that having things isn't everything?