Have you ever wondered where cork comes from? It comes from the bark of the "Cork Oak tree" (tree species name - Quercus suber). The cork bark of this species of oak tree (yes it does produce acorns) can grow to become two to three inches thick. Normally however it is harvested when it is about one or one and a half inches thick. In the image above you can see several different layers of the bark, the result of being harvested several times.
Only the bark on the main trunk and lower branches is harvested using special tools that peel off large sections of the outer bark without damaging the tree. What is left is the dark inner bark that you can see on the tree below. The bark takes about 7-10 years to grow back to a thickness that can be harvested again.
The picture below is a piece of the cork bark that I found laying on the ground. This particular piece was almost two inches thick. One curious thing that I have found with the bark of this oak tree species is its seeming resistance to rotting. I have often found sections of branches laying on the ground where the wood has rotted away leaving a "tube" of bark that does not seem to have rotted at all.
One interesting use for cork bark in Spain is in the tradition of making nativity sets. In almost all nativity sets you will see sections of this bark used to make the homes or stable or even caves. Keep in mind that in Spain nativity sets tend to be very elaborate and large "model villages" with Bethlehem, Roman forts and the countryside represented.