The UFC Octagon is made primarily from metal. There are a number of aspects to the Octagon and its structure, with metal as the chief material.
The ring posts that are inside the Octagon are all made from metal, as per the official UFC website. The fence posts that help create the structure are also made from metal. The fencing of the Octagon itself must be made from a material that prevents competitors falling out of the structure. If that material is metal, it must be padded and covered so that it is not abrasive to the competitors inside the Octagon.