A diamagnetic material produces a magnetic field that opposes an externally applied field. Normal paramagnetic materials like 'soft iron' produce a south pole when presented with a north pole, and hence attraction occurs. Instead, diamagnetic materials produce a north pole when presented with a north pole and hence repulsion occurs.

The upshot of this is that stable levitation is possible using diamagnetic materials.

Carbon, water, silver and bismuth are all noteably diamagnetic. Even people, since they are mostly made of water!

A small frog has been levitated using this principle inside an enormous Bitter electromagnet. The frog was perfectly weightless as the levitation occured at the atomic level and was well distributed within the frog.