I believe the answer is limestone..
ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ、ヽヽ`ヽ`、ヽヽ`ヽ`、`、ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ`、ヽヽ༼ຈ
ل͜ຈ༽ノ☂ ɪᴛs ʀᴀɪɴɪɴɢ sᴀʟᴛ! ヽ༼ຈل͜ຈ༽ノ☂ ヽ`ヽ`、ヽヽ`ヽ`、`ヽ`、ヽヽ`ヽ`、ヽヽ`ヽ、ヽヽ`ヽ
The answer is cornea…..lens. Most bending of light entering
the eye is bent by the cornea. The lens of the other hand adjusts this bending
further, to focus it squarely on the retina. This is why the focal point of the
lens can be adjusted by changing its shape.