Good news: Kryptonite is real.
Bad news: It's not green.

Yet another in a series of science mirroring science fiction:

Researchers from mining group Rio Tinto discovered the unusual mineral and enlisted the help of [Dr Chris Stanley, a mineralogist at London's Natural History Museum] when they could not match it with anything known previously to science.

Once the London expert had unravelled the mineral's chemical make-up, he was shocked to discover this formula was already referenced in the literature - albeit literary fiction.

"Towards the end of my research I searched the web using the mineral's chemical formula - sodium lithium boron silicate hydroxide - and was amazed to discover that same scientific name, written on a case of rock containing kryptonite stolen by Lex Luthor from a museum in the film Superman Returns.

The new mineral does not contain fluorine (which it does in the film) and is white rather than green but, in all other respects, the chemistry matches that for the rock containing kryptonite."

Now, about that whole "flying man of steel" thing...