From the BBC:

A new mineral matching its unique chemistry – as described in the film Superman Returns – has been identified in a mine in Serbia.
According to movie and comic-book storylines, kryptonite is supposed to sap Superman’s powers whenever he is exposed to its large green crystals.
The real mineral is white and harmless, says Dr Chris Stanley, a mineralogist at London’s Natural History Museum.
“I’m afraid it’s not green and it doesn’t glow either – although it will react to ultraviolet light by fluorescing a pinkish-orange,” he told BBC News.