Yeah... I think it's Jesus's crown of thorns and a part of the original cross. Some people claim that's stored in the fallen spire, but I think that's actually stored in the treasury in the basement. We'll see if they survived.
Actually, the only thing that wasn't stolen at the Louvre's Apollo Gallery is the Regent Diamond, figure no one would take it because something to do with a curse.
I've also found that the two thieves were leaving for Algeria, thanks of a trace of DNA evidence recovered from a helmet and their past in burglary cases according to the police.