What is the stopper used to seal wine bottles traditionally made of?

Wine corks are a stopper used to seal wine bottles. They are traditionally made from natural cork (bark of the cork oak), though synthetic materials can be used. Common alternative wine closures include screw caps and glass stoppers. 68 percent of all cork is produced for wine bottle stoppers. Corks are manufactured for still wines as well as sparkling wines; the latter are bottled under pressure, forcing the corks to take on a mushroom shape. They are fastened with a wire cage known as a muselet.

Source: Wikipedia