You'll Believe a Man Can Fly (and die)

It turns out kryptonite is real, sort of. Some London researcher discovered a mineral in a mine in Serbia, and the chemical composition of this mineral perfectly matches the chemical composition of kryptonite. In Superman Returns, a rock of kryptonite is on display in a museum, and the film lists the chemical compound as being sodium lithium boron silicate hydroxide.

http://news.bbc.co.uk/2/hi/science/nature/6584229.stm

The article says it's white and harmless, but clearly the author does not know the dangers of white kryptonite.