Oh so many to choose from!
I went to the natural history museum in Geneva a couple of years ago (while I was working there - made a very enjoyable break on a rainy weekend). One of their permanent exhibits was of minerals under UV light - showing their fluorescence. Some were a-ma-zing!
There's a little selection in the piccy here - http://en.wikipedia.org/wiki/Fluorescence
(click on the piccy to enlarge).