There are quite a few materials that are diamagnetic (repel magnetism), some are; water, bismuth and forms of graphite. The levitation jig makes a magnet hover between 2 blocks of diamagnetic material.
Whilst researching the levitation jig, I found this page GerAx links that has all of the info needed.
The complete writeup is here; Lego Diamagnetic Levitation Jig
Please remember, your safety is your responsiblity should you choose to replicate
anything you find on stop4stuff.com
This page is part of a frameset, Click here to view the page as it is meant to be seen