Eyeballs are a special part of our body, with many unique properties. This video discusses what they're made of: collagen. Collagen is a critical structural protein that is one of the major components of connective tissues like skin and tendons; that makes it one of the most abundant proteins in the bodies of many mammals.
The eye has a layered structure. The sclera is the outermost layer, encasing everything except for the cornea. There are several more layers under the cornea, before reaching the vitreous humor. That gel-like substance fills most of the eyeball's volume. At the back of the eyeball is the part that allows us to see: the retina.