Looks like a cheap agglomerate cork, but it isn't. It's one of Oeneo-Bouchage's new Diam closures. They're revolutionary. Oeneo (then Sabaté) launched the Altec a few years back, which was a technological cork that was part cork granule and part synthetic - the problem was they ran into horrible TCA (the compound found in cork that gives musty off flavours) problems. They responded by devising a washing procedure that involved critical point carbon dioxide (read here for more on this). The result is Diam, a virtually taint-free closure with excellent mechanical properties, which is now being used by several high end producers with good results.
(All photographs are ©Jamie Goode and must not be republished in any form without permission.)