Street Magic by Tamora Pierce
Plant mage Briar Rose has saddled himself with a teaching job. A young stone mage, Evvy, who he spotted working magic in the market - when she had no idea that she was doing anything more than polisihing crystals for a few coppers. Unfortunately for Briar - who doesn't want to teach anybody anything - he's got too much curiosity in him, and he can't just ignore it. He doesn't know what he's letting himself in for...