Cork

Of course, it’s the object that seals a bottle.

But, in particular, cork (the natural kind, not synthetic) is the bark of the Cork Oak tree (Quercus suber) grown primarily in Spain and Portugal. The bark can grow two to three inches thick, and its unique physical properties make it ideal for stopping a bottle: it has a honeycomb cell structure, is elastic, waterproof, lightweight and fire resistant.

Detail of bark layer from the Cork Oak tree. (tree-species.blogspot.com)

The bark is stripped from the tree, the corks are stamped out, and then cleaned and imprinted. The tree’s bark does grow back, and is ready for a new harvest of corks about nine years later.

(from uncork.biz and Great Wine Made Simple, Andrea Immer Robinson)