Well everybody has been talking and posting about this one, so I better get my 2 cents in as this is Atomic Romance. It seems a white chalky substance found in a Serbian mine has the same composition as the fabled science fiction substance that can bring Superman down, Kryptonite. As noted here.
While kryptonite is often depicted in Superman comic books and movies in the form of green crystals that possess the power to render the superhero weak and powerless, the actual mineral is white, powdery and ultimately harmless.
"The new mineral does not contain fluorine and is white rather than green, but in all other respects the chemistry matches that for the rock containing kryptonite," said Dr. Chris Stanley, a mineralogist at London's Natural History Museum.
Ahh... but you see, in the comics there are several different kinds of kryptonite and yes there is white kryptonite. It doesn’t hurt Superman actually, it’s a super plant killer. So, maybe we'll get the glowing, green, poisonous rock if we add a little fluorine right?… Ahh no. But, according to the BBC here, at least they’re giving the stuff a different name, Jadarite.
They have to as krypton is an actual substance not found in Jadarite, but that's OK. Let's keep sci-fi and reality separate in this case
Here’s a news report by the CBC:
And this one with a little more professional explanation