Kryptonite?
 

What's with the green diamond the crooks are trying to steal in Michelangelo's micro issue? The cop sais their boss wants it because he's obsessed about it's radiation properties, prolonged exposure etc...

Is this intended to make us think they're working for Lex Luthor and we're going to get a Superman crossover? Probably not.

But what do you guys think it's role is? Or will it remain just that, a parody?

Mutagen ingredient or something along that line

Mirage
 

Do you mean some Mirage issue?

No IDW's Michelangelo micro.

Considering this is an IDW comic, and Lex Luthor is owned by DC, no. It's not Lex.
Krang probobly needs it for more mutagen or something, so he hired some thugs to subtly steal it for him.

Not at all trying to say it is Lex, mutagen ingredients is much more likely, but an IDW and DC crossover isn't unheard of. Look at the Star Trek and Legion of Superheroes crossover.

You guys do know I mentioned Lex Luthor in jest right?

I think they're actual boss is Shredder, though he could probably get his foot ninja to steal it. Perhaps Krang does make more sense.