Erik clearly wasn't buying their denials and story that they had just the one gem. So much so that he lightly pointed his wand at the door and turned his wrist, so the lock clicked shut.

Locked in now, girls. What are you going to do about it? Erik, meanwhile, lounged back in his desk chair quite comfortably. He folded his fingers into a steeple and tapped his fingertips together thoughtfully. Rap a tap tap, he had them in a trap.

"You rehearsed vhat story vell, Alexa, but I don't believe it. I know vhe goblins vere distracted as they collected vhings from around vhe castle...it vould have been incredibly easy for anyone to slip a few gems out of sight, vhen they all spilled from vhe hourglasses. You're a Gryffindor, and a feisty one at vhat. I don't believe you vould haff been scared away from stealing some shiny, valuable gems just at vhe sight of a sword."

Erik slowly shook his head. "That topaz cannot be vhe only gem vhat escaped. Now you can either come clean vith me, and vhis can stay between us, in vhis room, or....vell I can send for vhe Headmistress now. I'm sure both she and vhe Gringotts goblins vill be interested in hearing just how much you stole from vhem. So," he stared the girls down with his unblinking blue eyes, "Vhat vill it be?" The truth or the punishment? He really hated to punish like-minded, favourite students.