"Merlin's beard, girl," Snape growled.  "I'm a master potion-maker.  Have you learned nothing in six years of magical learning?" He muttered something under his breath that Hermione thought sounded like "Ghosts are transparent. Merlin help me."  Hermione bit back a snicker, remembering that answer Harry had given him, though she could no longer remember what the question had been.  He held up a small red stone that reflected the flickering firelight and Hermione gasped.  "Ah. You recognize it," he said.  "Perhaps all hope is not lost."