Kryptonite was never green, it is actually white! Oh! Superman, falling for mind games, are we not?
Now, seriously, a mineral that matches kryptonite's chemistry, as it was described in Superman Returns (sodium, lithium, boron, silicate, hydroxide), was found in Serbia; the only difference being that kryptonite is supposed to have fluorine, while the one that was found in Serbia doesn't.
http://news.bbc.co.uk/2/hi/science/nature/6584229.stm <--- Full article.