Cleaning a Dull Geode?

Going through an old box that had been in storage, I came upon a geode that I hadn't seen in 20 yrs. I remember the center crystal was bright and glistened, almost like amethyst. Now, it is dull and looks only like a rock. Is there anyone out there, who is familiar with geodes, and knows what to use to clean them to restore the luster? All replies are appreciated.

By Luann DeLuca from Corning, NY