The materials from which the tweeter is made. Common materials include polypropylene, silk, cloth, and paper. Many contemporary tweeters are now manufactured with composites that may include any combination of aluminum, titanium, Mylar, and other ingredients, along with the more traditional polypropylene.
As tweeters move extremely fast (over 20,000 cycles per second in many cases), It's essential that they be made of very lightweight materials.