The cornea is a clear transparent structure, which appears to be variously coloured as it reflects the colour of the underlying iris. It's accounts for about 60% of the refractive power of the eye! Whereas the human lens accounts for only 40!!
The functional capability of the cornea relies on its ability to remain clear and transparent.
A wide variety of corneal diseases can impair the corneal transparency, like ocular trauma, ulcers on the corneal surface, and chemical injuries to the eye, congenital defects, and dystrophic conditions.
A diseased and scarred cornea, which cannot be treated with medical treatments, will require surgical intervention.
This surgical procedure is called a keratoplasty. Either the entire thickness of the cornea is replaced or layers or "component" of the cornea may be replaced. This component keratoplasty is an emerging new technology (DSAEK, DMEK, DALK).
What many don't know is that when one donates their eyes, it's the cornea that is used to restore the vision to two other blind people.